Your worry is indeed honest worry. Probably, the back discharge you see could be deep crown or chocolate colour. Such discharge is due to old or stale blood. Will you please closely observe the colour of the discharge and whether it is thick? It is likely that your uterus has not come back to its pre-pregnant state by now. Usually by the end of 3 months, uterus attains almost its pre-pregnant size. If it does not, it is called as sub-involution of the uterus. In such condition, due to some what relaxed state, the uterus bleeds little bit. Because it has remained bulky, the blood can stay inside for some time and so when it flows out it is stale blood having deep brown to chocolate colour. The birth control pills also my paly a role in relaxing the uterus due to Progesterone in it. Are you not feeding the baby on the breasts? Breast feeding does contract the uterus and helps to come back to its pre-pregnant state. I would advice you to take an opinion of an obstetrician who will examine you clinically and advice properly. You may require some medicines which make the uterus contract.