I get this after riding a few hours on a road trip in the car. I think it is related to the actual muscles in my stomach - like you, I have a tough time standing up straight without pain after a long ride or sitting in the car, but once I stretch out, it's okay. Pilates helps. I think when we're on the bike, in combination with all the other things our stomach is doing (digesting, our pelvic muscles dealing with the bladder, etc.) the stomach muscle contract or something... and cramp. I cut the elastic on my bike shorts and that helps a lot.