When I was pregnant with my son, I had horrible sharp pain in my right butt cheek. It turns out it was the SI joint relaxing and the muscle getting pinched inbetween the two bones. Ouch!! Two things helped: one, don't sit in the cross legged/'Indian' style position, it makes it worse, and two, physical therapy. Physical Therapy helped a ton and I would highly recommend it. It got the pain to go away completely thru heat, massage (yes, on my butt) and exercises.