From: Rabbi Moshe Sofer
To: Rabbi Moshe Aryeh
A woman who consistently sees blood during marital relations, but never on the night of tevilah itself—only on the third or fourth night after immersion—has established a fixed veses pattern relative to tevilah. The Chasam Sofer rules that the blood is attributed to the veses rather than to the marital act, permitting her to her husband on tevilah night and subsequent days until the pattern-day, after which she is forbidden until she immerses again. He additionally advises consulting physicians to determine whether the bleeding originates from a vaginal wound rather than the uterus, which would resolve the issue entirely.
Source: Shu"t Chasam Sofer YD §174