What Causes Tumor On Buttocks?

My daughter is 45 and has just found a tumor on her buttocks. She was over weight for years, so no clue how long it has been there. After losing 100+ pounds in the 18 months after surgery, she experienced pain. She works at Duke so access to good medical care. Has an appointment with a specialist next week. In all my years on earth, this is a new medical diagnosis. Please advise. Thanks from a caring mom!