What Causes Brown Vaginal Discharge And Cramps?

hi im 16 and im going on a holiday thursday where im going to be swimming and i usually have my period at the end of the month and ive only taken the pill a few times before this to get rid of it because of school swim camps and such but now a few days after taking ive got brown like discharge and some brown blood and massive cramping what is this and no i couldnt be pregnant i hqvent had sex yet.