Boyfriend came inside of me on my period and i took plan b the next day just in case. is that bad? will plan b mess up my system if i wasn't pregnant?

A little bit of bad: The little bit of bad is having to take Plan B because Plan A didn't happen. The question to ask oneself is, "What's my Plan A?" or "Why did I end up having to take Plan B?" Plan B can alter the menstrual bleeding for the cycle in which Plan B was taken, and sometimes the next cycle too. That's not harmful, though. One should be sure to check a home pregnancy test about 3 weeks after sex.