I have a red lump on my bum. It's about 5mm under my skin and about 3cm big and it's painful to touch or sit on. There is nothing on my actual skin on top of the lump except for a red color that extends beyond the lump by a few millimeters. I am positive it has nothing to do with a hair follicle. It has gotten bigger gradually over the past week and I'm not sure if it could be a cancer of some sort or just a smaller problem. If it is a cancer does anyone know what kind it would be? What else could it be?

Thanks