Idk what my problem is...

Ok so I’m sorta bad at explaining things so just bear with me here please.

On Saturday September 23, I had sex for the first time. We forgot to use a condom but he pulled out. However, to be safe, he bought me a plan B pill to take on Monday because we weren’t able to see each other on Sunday. So I took the pill as planned. I didn’t really have any side effects of the pill so I figured I was in the clear. I started my period a week earlier than usual on Saturday September 30, but I’d rather have it too early than too late. Despite it being like a regular period that lasted a week, I still have brown discharge with a tiny amount of blood in it currently. I also feel like I’ve been getting more stomach aches than usual. My chances of pregnancy were very very low, and I got my period, so I don’t really think it could be that. What could be the problem though? Please help.