Hello.I'm curious. I just started my period a day ago and this is my second day being on it. I just had sex tonight and he cummed in me while I'm on my period. I'm on the pill but is there a chance I might get pregnant? I'm only 16.

Yes there's a chance, no matter how small.

It is a matter of probability. These are ball park numbers resulted from Google.

On average there is a 20-25% chance of getting pregnant when you don't use protection. If you use condoms, which reduced that number but about 80-90%, you go down to a 2-3% chance of becoming pregnant. When you use a hormonal pill, it reduces it by 95%, so about 1% chance. Assuming that you're following the directions on your birthcontrol completely, then your chances are the same as if you had sex when your would be ovulating if you weren't on birth control.

The mechanics. Most hormone pills prevent pregnancy by convincing the body it is already pregnant. So no need to waste resources by ovulating and close up shop because it is baby making time. At the end of the month things wear off and the body sloughs the lining. So when you have sex on your period a lot of the semen should wash out but some will stay and swim up and around your tubes and escape the period. So if you didn't follow the instructions completely and ovulated early, the sperm could impregnate you.

TL;DR: You could because pregnant but it is the same chance throughout your cycle. Assuming, and that is a big assumption, you take the pills correctly.