really bad period or miscarriage?

Hopefully you were told by your doctor NOT to have unprotected sex in the first 6 months after delivery? As women are VERY fertile after the delivery of a baby. IF the pregnancy test said no then it's no. Depending on if you are breast feeding etc. then I suspect that this is just a heavy period - due to the build up after the uterine trauma of birth. It takes a while for the uterus to build up a lining again, after the birth of a child. You should NOT be having unprotected sex till around 6 months - as your body is more accepting of a zygot and embryo - just ebcause it is familiar to it (due to it having a baby). Think of it like this, you need a security pass to get into your work every day. Then the security guard starts recognizing you, soon you wont need a card as he recognizes you to be above board and no trouble - same thing with your body and an embryo! You need to let your body heal and get back to pre baby body before having another child!