How am I supposed to tell if I'm pregnant or not when I'm bleeding?

Last week (March 13-20), my boyfriend and I sort of had sex. I say sort of because, though he was penetrating me, neither of us actually finished, since I wasnt on the pill and we didnt have a condom. I read somewhere that it’s possible, though highly unlikely, to get pregnant from pre-cum, if the man has ejaculated recently and has not urinated. I started my period on the 22 or 23 (can’t remember exactly), a few days ago, which is on time from my last period. It was normal for me (fairly heavy most of the time and normal flow the other times). The problem is it seems to have slowed, almost stopped. It’s only the second or third day. Sometimes, with me, I will think I’m done, but I’m actually not. The problem is my periods are never constant. They are always switching between being at the beginning of the month, to the middle of the month, to the end of the month (I’m 19, almost 20). I have had periods before that last only three days or so, but it’s rare. They normally last around 5 days. I dont really feel any different, but I’m starting to get a little nervous. Is it at all possible I could be pregnant? Everything I’ve read says that if you bleed during the beginning of the pregnancy, it’s light spotting. I didnt know if it was possible to experience heavy bleeding like a normal period. Or is it possible that, since we did that, it could have changed how I experience my periods? Please advise!