If the lump is ON the scar line:

It formed there because the scar wasn't taken care as well as it should -OR- the doctor wasn't a professional and he pulled back much tissue when he was stitching the area there when he circumcised you.

It ISN'T anything bad nor something that need any cure. But for aesthetic reasons, i suggest to go to an urologist and tell him to remove the extra tissue that formed there - I would have done the same if i had the same problem-.