I Have Had A "blood Clot" Looking Bruise Since I

I have had a "blood clot" looking bruise since I was a kid. It now causes moderate pain, and it is hard-like, and I wanted to know what I can do about it, or what type of doctor I may need to see about it. It is always looks like a dark purple or almost black looking color. Do you know what caused it, and if there is anything I can do about it. It is on my right buttock cheek.