Please answer

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 was a lot at first but now there's barely any. 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. Is it my period, implantation, or side effects? Also if it's implantation how long after taking the pill should I wait to test? I really hope I'm not pregnant.