What Causes Dark Spotting, Cramping And Acne Before Periods?

i am currently on orthocylcin birth control. have been for almost two years now. at the end of my period last month i had sex with my boyfriend twice, three days between. becuase of condom failure i took two doses of plan b right after sex (three days apart) now on the 2oth i started to bleed. however it was not like a normal period, it was very dark brown, almost black. i am cramping and breaking out like i have during my period. i did see a little red but very little. now i am supposed to get my period in 3 to 4 days, but now the bleeding has seemed to have stopped. what is goin on with my body?