Sex while on your period

So, me and mg boyfriend had sex yesterday. (Saturday) but I started Friday. We went for about 20 minutes and after I took the condom off with some toilet paper and I noticed that there wasn't any cum in the condom. He said he finished. But I'm really scared, could I be pregnant? I was thinking maybe the cum dripped out of the condom when I took it off or it got out of the condom on to the toilet paper. I have no idea. I'm really scared