"Butt Cramps" are usually caused by spasms of the piriformis muscle, which connects the outside of the hip to the sacrum.  It can come from overuse of the muscles or from prolonged periods of inactivity.  If the pain becomes very frequent or is unbearable, ask your health care provider to prescribe a pain relieving medication that is safe for use in pregnancy. Some other thing you might try are: changing positions so as to put less strain on your muscles 'Heat (a heating pad, hot soaks, or a warm bath); Avoid sudden movements, (sit down or stand up slowly);  Pay attention to what things seem to trigger or set off episodes of pain. Then you can avoid those things. Be sure to let your doctor or midwife know how bad it is and call them if you experience an episode with new symptoms, or is worse that before.

Here is a link to a website that contains some exercises that may help:

http://lifestyle.iloveindia.com/lounge/butt-cramps-1908.html