Implantation, period, or side effects?

I had sex last Sunday and he came inside of me. We used a condim but I was still worried. Monday I took the plan b pill. Today I wike up to some blood in my underwear. There wasn't a ton but enough to to need a pad. There's not a lot on the pad but when I wipe there is a lot. I'm scared that I am pregnant. Should I be worried or what? I just got off of my period on like the 29th or something. I'm not supposed to start again until the 24th.