What Causes Vaginal Bleeding Before Period?

Hi,I have been on Tri Cylen Lo birth control pills for more than 2 years now. I was recently fooling around with my long term boyfriend and while we was fingering me I experienced vaginal bleeding. I didn't feel any pain or discomfort and he doesn't have long nails or anything so I seriously doubt he cut me. And I'm not expecting my period for another week. I would like to know, what is the most likely cause for this? Should I be really worried?