I had sex on my period, and it stopped completely. Could I be pregnant, or is it too soon to tell? I’ve never had this happen before, and I’ve been lactating. Maybe it’s just nerves?
What day of your period was it when you had the unprotected sex and how long is your cycle? Those actually play a pretty big role in whether or not you’re chances are near zero - or higher.
For example, Very rarely, a woman can get pregnant if she has unprotected sex on the first day of the period. The chances are more likely though, for a woman who has a 20 day cycle because the egg is released around the seventh day; and the most fertile in this case would be days 5, 6 & 7 of her period cycle.
Sperm can live inside the woman’s body for up to 5 days.
So to answer your question, yes you can get pregnant if you have sex while on your period.
Sex does help with cramps and does shorten your period. Not sure how but that is what i have heard and does actually happen with me 
But for your main question - you can fall pregnant at any stage of your cycle, although your period stopping suddenly is not pregnancy related. Take a test in 2-3 weeks x