Suggest Treatment For Brown Vaginal Discharge With Stomach Pain

hi i have been dieting for the last 2 month and have lost neally 2stone which isnt bad seen as i was 16 stone. i am on the pill and have never had any trouble with my periods. last month i got the stomach pain, back ache and breast pain only to be disappointed with a thick dark brown gooey discharge instead of my period. i waited til this stopped, which was a few days and began my new cycle of the pill. however i missed one on saturday, continued takin them the days after, am nowhere near the end of 21 pill cycle and am again having this discharge with the same pains.