I Have A Large (1.5" Across) Lump On My Lower

I have a large (1.5" across) lump on my lower right buttock, almost at the crease of my leg. It's painful but has no head. I can actually pick it up under the skin and get my fingers underneath it a bit. I'm 57, 5'6" tall and weight 159 pounds. Is this a cyst? I've had it for about 3 months but it grows and then seems to shrink down for a bit but never goes away entirely.