How do you know Plan B worked?

I had my period on may 27 2011 and we had sex twice on the 7 I think it was, anyway I took my plan b on Sunday morning around 9:30 but the second pill was misplaced. We have continued to have sex, and I have had breakthrough bleeding start 5 days later. Sorry ahead of time but the bleeding has been medium to light flow and BRIGHT red but it seems watery and slimy, so what I guess i am trying to get at is I took it 3 days before I was suppost to ovulate, does that mean it didnt work? Could I hae still gotten pregnant in the mean time? After taking the pill we have decided that maybe we will try for one, but the whole breakthrough bleeding threw me off alot please help me understand how this works.