Cramps come from prolonged contraction of muscle fibers. Cramping often occurs during or right after exercise, when you sweat a lot or have a fever, from repetitive motion activities such as writing (writer's cramp), and as a result of poor blood circulation (sometimes associated with leg cramps). Some cramp-reducers include:

Massaging the affected area, which often reduces the pain and duration of the cramping.
Adding more potassium-rich foods to your diet (such as bananas and nuts), which could help alleviate and prevent future cramps.
Medicine can make a difference if you're chronically cramped.

Since cramp may be more of a convenient term than an accurate description of what's up with you, a visit with the doctor may soothe your bum much sooner than bananas and a massage.