What Causes 6 Months Bleeding After Abortion?

Hi i have. A question i have had 6 months of menstral bleeding that followed an abortion. I have had an ultrasound to check for retained peices but there were none, but they found a cyst on my ovaries. Does that have anything to do with the bleeding or can u suggest something to do cause the doc say everything looks fine and they dont have anything to do with bleeding