Bloody Discharge?

HELP! I've been on BC for a year now. Had spotting the first few months and then it stopped. I've been having unprotected sex with my fiancé for a year now and BC has always seem to come through.

I had unprotected sex with my boyfriend on Saturday. Afterwards, there was a smell but it went away after i took a shower and never came back so, i didnt think much of it.

I've been spotting since Sunday. I have the patch on right now. Does anyone know what this might be? pregnancy? Infection? Or just a common side effect of BC? HELP!