irregular bleeding?

I had my period the 3 weeks ago & it was a normal one (lasted as long as usual). during that 3 weeks I had sex w/o a condom but while on birth control. I didnt think I took my birth control correctly so I got Plan B and took that. about 2 weeks later, I got what I thought was my period, out of the blue. it was lighter than my regular period but heavier than what I would think spotting is (i used a super tampon for the 2 days and it soaked through twice) and only lasted 2 days. someone please give me some advice on what u think It could be, if anything at all. (PS, I am not supposed to get my period while on this birth control at all)