Wait for period or Plan B now?

I have unprotected sex but he was ther for a couple second before I actually realized what I was doing. I don't exactly know if he came or not. my period is supposed to start in 12 days so I don't know if I should wait for my period or take the plan B pill. I've heard that the plan be pill can totally mess up for vagina for a long while and I'm worried what to do..