Plan B Help

I was on my period until November 4th, I then had sex the next day and my boyfriend and I got a little carried away and he ended up not using a condom or pulling out in time. So I took a plan b the very next morning. It's now about a week later and there's blood in my underwear. Is that my period?? Has this happened to anyone else? 
I've taken plan b before and it's never done that.