Bleeding in between periods??

I've been on the pill for almost a year, and my periods have been very regular as a result (they've never really been irregular anyway). Lately, I've been bleeding/having discharge in between periods! It's to the point where it'll mess up my cute panties and it comes unexpectedly, after my last period has ended and before my next one is supposed to start. :-( What might this be? Does anyone have any suggestions for things I should do?