There are multiple causes of pain in the hip and buttock region while sitting and, if these symptoms have been going on for a long time, you might want to go see your primary care doctor
If these are true cramps, rather than just discomfort or pain, then one possibility is that your muscles are too tight or fatigued from physical activity. For example, if you tend to work out vigorously, this is probably what is going on, and you can likely prevent the issue by backing off on your work out routines a bit or by stretching better before and after working out. Dehydration can be another trigger for muscle cramps, so make sure you are drinking plenty of liquids. Finally, in some cases, imbalances in various levels of electrolytes in the blood stream can be a cause of cramps, so if they continue your doctor may want to check some basic blood tests to rule out this possibility.
If on the other hand what you are experiencing is actually discomfort or pain, rather than cramps, this opens up another series of possibilities, including arthritis
, nerve problems, and bursitis. The first step is seeing your primary care doctor.