The other girls have pretty much covered what i would suggest, the sacroiliac pain is felt around the sacrum (The large bone at the bottom of the spine, above the tail bone)
It could be sprained or your pelvis might be out, this also causes sacroiliac pain.

It could also be sciatic pain, which sometimes is and sometime's isn't felt in the leg also. It tends to be on one side, but can be in both.

It could be a compressed nerve because of the tightness in the gluteal muscles. That major muscle someone mentioned, the piriformis, can give you some real nasty pain when it is locked up.

A massage can relieve muscle tightness which can help sciatic pain and general glute tightness, and can also help relieve the pain associated with the sacroiliac joint. A chiropractor (find a good one) can help with structural problems in the hips and pelivs. Cause of the changes in your body and everything loosening up to accommodate this kind of pain is quite common. Use a heat pack on the area to warm up the muscles. It can help to sit or lie down and place a tennis ball under the muscles to help relieve the pain, kinda roll your body over it.

Hope some of this helps!!