Gemara

(Rav): Dam Tohar comes from the same source as Dam Tamei (of a Yoledes, Nidah or Zavah). The Torah decreed that during Yemei Tohar it is Tahor, and at all other times it is Tamei;

(Levi): Dam Tohar comes from a different source. During Yemei Tohar, the source of Dam Tamei is closed, and the source of Dam Tohar is open.

59b (Mishnah - R. Meir): If a woman urinated and saw blood:

If she was standing, she is Teme'ah; if she was sitting, she is Tehorah.

R. Yosi says, in either case she is Tehorah.

Question: What difference does it make if she was standing or sitting?

Surely, the stringency of standing is that the urine goes to the Makor (before coming out), so the Dam came from there. Also when she sits, (if she does not allow the urine to spurt) it goes to the Makor!

Answer (Shmuel): The case is, she allowed the urine to spurt.

Question: Perhaps after the urine spurted, blood came from the Makor!

Answer (R. Aba): The case is, she sat on the edge of the Kli. Had the blood came after the spurting stopped, it would have been found on the Kli.

(Shmuel and R. Aba): The Halachah follows R. Yosi.

Rishonim

Mordechai (Shevu'os 735): A woman is believed to say that she has a wound in that area that emits blood. This is only if she knows that the blood came from the wound, but not if it is a Safek. At the time of her Veses, even if she knows that it emits blood, she may not attribute to it. If not, she would never be Temei'ah! However, she may attribute stains to a wound, even if she does not know whether the wound emits blood. Kesamim are mid'Rabanan, so Chachamim are lenient about them.

Terumas ha'Deshen (Pesakim 47, brought in Beis Yosef YD 191 DH v'Chasuv): If a woman knows that she has a wound, but she does not know whether it emits blood, we do not attribute blood to it. The Mordechai explicitly says so. However, in the case I was asked about, there are two reasons to permit. 1) She finds the blood and fine hairs only in this one place on the side. It seems that the opening of the Cheder (the Makor, the source of Dam Nidah) is in the middle of the Prozdor. A Beraisa (66b) teaches that if a tube was inserted and blood is found on the end of a stick passed through the tube, it is surely from the Makor. If not, it is surely from the walls. This implies that in the latter case, she is Tehorah. 2) Here surely it is not from the Makor, for she feels pain when she checks that place, but not when checking other places.

Poskim

Shulchan Aruch (YD 191:1): If there was blood in a woman's urine, whether she urinated standing or sitting, she is Tehorah. Even if she had a Hargashah and shook, she is not concerned for it, for she felt the urine, which is not from the Cheder. The blood is from a wound in the anus or the kidney.

Chasam Sofer (2 YD 154 DH Nosaf): The Radvaz (1:316) says that sometimes blood from a wound in the anus cannot leave from there, and it leaves with the urine.

Rema: Some say that we permit her (in every case) only if she sits and urinates. If she stands, if the urine streams into a basin and blood is found there, she is Tehorah, but if it drips onto the rim and blood is found there, she is Temei'ah. Since the place is cramped, the blood backs up to the Makor and brings blood. Some say that even if she sits we permit only if the urine streams and blood is found in the basin, but if it is on the rim, she is Temei'ah. If she was standing, in every case she is Temei'ah. This is the custom. This is if it happened once. If she regularly sees blood in her urine and feels pain when urinating, like the sickness called Herin Vind (perhaps this is a wound in the groin - the Magihah in Chasam Sofer YD 144 DH Aval), we can permit in every case, for there is evidence that there is a wound that pains her when urinating, and the blood is from it. Even if she found blood after urinating, when cleaning herself, she is Tehorah. Since she has the pain and she finds blood only after urinating, surely it is Dam ha'Makor.

Gra (6): See 187:7 (brought below).

Pischei Teshuvah (3): Tzemach Tzedek says that we permit one who sees blood in her urine only if she finds blood also in the urine in the basin, and it hurts when she urinates. Then we permit even if after urinating she finds blood on the Bedikah cloth, for we attribute the blood to what she found in the urine, which is surely not from the Makor. Alternatively, if she did not find blood in her urine, but she has great pain urinating, and she finds herself clean before urinating, and puts in a clean wad and urinates (and sees blood in the urine). We do not permit her if she does not feel pain when urinating, even if she feels pain beforehand and afterwards. Sidrei Taharah disagrees. He says that even if she does not find in the urine, and has no pain urinating, just immediately afterwards, and she finds blood, she is Tehorah through the Rema's Bedikah. This is clear from Chacham Tzvi 73. He later brought from Shevus Yakov 2:76 that l'Chatchilah one should be stringent, and one should deliberate this law before ruling on it.

She'elas Ya'avetz (1:55): I was asked about a woman who cannot be Metaher herself to her husband, for she sees blood after urinating. I could not find a Heter. This is unlike one who sees due to Harin Vind that the Rema mentions. All Raboseinu agree that even if she has a Veses, we may not attribute to a wound unless we know that it emits blood, or there is an awesome proof. In such a case, only for one who sees blood due to relations, Chachamim were lenient to say that a wound is prone to emit blood, because the Ever presses. This is lest she be forbidden to her husband, and only not at the time of her Veses.

She'elas Ya'avetz: We do not attribute to a wound if we cannot clarify its location, unless there are awesome proofs, e.g. a great pain urinating and there is Chol (presumably, this means small particles) amidst it. Then it is clear that it is from the kidneys. Or, if a scab comes with the blood and she is sure that it is from the side and not from the Makor (we attribute to it). However, if a doctor says that there is a wound above, under the chest, even if we believe him that blood flows from it when it is open, since he admits that the blood comes through the Makor, all agree that it is Tamei blood, and forbids to her husband. The argument about whether the place of the Makor is Tahor or Tamei concerns only blood from a wound in the Makor, for we know that no blood from the Makor is mixed with it. I.e. the wound is on the walls of the womb. If blood comes from above and passes through Varidim (veins) of the Makor, from which blood of the Veses flows, and through there she becomes Nidah, there is no difference what causes the blood. Whether a wound in her body caused the Makor to open and emit blood, or if the Varidim opened by themselves and emitted blood, since it comes from her Makor, it is Tamei blood, even if blood of a wound is Vadai mixed with it and there are scabs with it. If blood comes with urine from the source of urine, it is Tahor, even if it goes back to the Makor, for we hold like R. Yosi. The same applies oppositely. Blood from the Makor is always Tamei, wherever it goes. We hold like Rav, who says that there is one Mayan, and the Torah decreed when it is Tahor and when it is Tamei. There is no other time when it is Tahor (except for Dam Tohar).

Shulchan Aruch (187:7): If whenever a woman checks herself in holes and cracks she does not find stains, only in one particular place on the sides, we attribute that it is from a wound on that side, and all the more so if she feels a slight pain when she touches that side, and she does not feel any pain when checking the other sides.

Pischei Teshuvah (37): The Noda bi'Yehudah (46) says that this discusses, like the previous two Sa'ifim, a woman with a wound. However, in Sa'if 5 it is known that the blood is from the wound. Here we are lenient even if we do not know that the wound emits blood, and even for a woman without a Veses.