What Causes Lump On The Buttock?

I have a lump the size of a dime about 1 cm deep that looks like a bruise. I've had it for 3 weeks now. It doesn't hurt until I push on it. I have no other symptom, not a pimple, it's on my left butt cheek about 2 in down from my waist line. Could it be cancer? Doesn't seem to be going away. There are no other symptoms