I have a golf ball sized lump in my right buttock. What's causing it?

Thank you for your question. I would first recommend that you talk to your surgeon regarding this concern. It is very possible that this is an area of fat necrosis or dead fat. This can occur after fat grafting. Please consult with a board certified plastic surgeon. 

Greetings. This is something you should have your surgeon evaluate.  There can be some lumps that develop but if things are worsening, seek medical attention and evaluation with your surgeon.  Best wishes.