hi i m 18, and have been on birth control for about 1 year now. i usually would get my period once a month, but starting a month and a half ago, i ve been getting my period every like 2 weeks or so. also, i find myself getting my period after i have sex, usually the next day. the day after i start to get period cramps, and i bleed brown/red blood. this happens for about 4 days and then this cycle repeats itself.