Maybe due to the development of your baby. Try to use pregnancy pillow while sleeping and sitting. It will provide you some relieve.
I had the same thing happen with my first pregnancy. Try doing some yoga. That's what I'm doing this pregnancy because my hip and lower back is messed up. Its been helping a lot!
Exercise or stretching? I had to take it easy with my dancing and also always sleep with a pillow between your knees.
Could be braxton hicks.
Could be your bodies reaction to a heavy baby sitting in there