Sholem asks:
If a woman would become a Niddoh at the onset of Peach how could she possibly bring a Pesach Sheini... Assuming a cycle of 28 - 30 days, she would become a Niddoh again !!
Sholem, UK
The Kollel replies:
Possibly she became pregnant and does not have a period.
KOL TUV
Dovid Bloom
Another possibility:
A more likely scenario would be that she has a longer cycle. The Shach Yoreh Deah 189:65 talks about a woman with a cycle of 35 days and this is not unusual.
Dovid Bloom