What Causes A Vaginal Discharge With Cramping And Uterus Discomfort?

I have been on implanon for two years after i had my son and havent had a period since i got it put in, and no spotting either. I have had NO complaints about my birth control. But just in the last few days ive been having this dark red-light brown color discharge, and been a little crampy and uncomfortable in the uterus area. I have unprotected sex but i doubt im pregnant. Its making me a little nervous because this hasnt happened since ive gotten implanon put in.