I fell down the steps and have huge hematoma and dent on left buttocks. Can it be aspirated and filled with my own fat?

Hematomas usually resolve on their own by a month. Residual lumps may be areas of scar tissue, fat necrosis, or encapsulated old blood. It is unlikely at this point that something can be aspirated, but an ultrasound can still be useful to determine what is in there and if it can be cut out. You may consider going to another surgeon or your primary care physician who can order imaging studies to evaluate your lump. Hope this helps!
Johnson C. Lee, MD
Board-Certified Plastic Surgeon