If what's happening is for sure your period (as in it has period symptoms and lasts about as long as your period does),it may just be a coincidence that your period starts during sex. Another possibility is that if you get very aroused or have an orgasm from sexual activity, that can appear to "kickstart" your period. Because arousal and particularly orgasm can cause uterine contractions, it's not uncommon to find that if that happens around the time a period is due, those contractions cause you to see flow earlier than you might otherwise or see slightly heavier flow, or more flow at once, afterwards.
I have a question. My cousin and her boyfrnd had a sort of moment. My cousins boyfriend after masturbating and ejaculation with the same hands fingered my cousin. She is a virgin. But she is late for her periods like 12 days . She had PCOD. But she had recovered . Last month also she had late periods. Is there possible of chancees of pregnancy?