What Causes Brown Spotting After Depo Shot And Intercourse?

Hi my name is mickey. I just recently went on the depo shot and was taking antibiotics for an std. I had sex last weekend and 2 days later I had a dark brown spotting I have also been cramping. It doesn t happen everyday just every other day. It s now a week after I had sex and it s still happening. Is this normal? Should I be concerned?