Lower back pain ..is it my period or am i pregnant?

I had an accident couple weeks ago and my bf semen got all over me the next day i took the plan b but this week we were having protected sex but he took the condom off and put it in for a while but he didnt ejacuate in me at all he ejacuated.30 secs after he pulled out i know its dangerous... but i have been waking up to really bad lower backaches is it my period because im still due this month or the beginning of the next month ..help??