I have the SAME thing. I wake up and it just feels like the gas is moving in my intestines back and forth and back and forth and it is so incredibly painful. This usually lasts for the same amount of time yours does (15-20 minutes) and once I start having a movement it is diarreha and the cramps are still excrutiating. I usually have another episode of the gas moving around for another 5 minutes or so and I have the same kind of movement all over again. Usually after the second one the cramps lessen and I can at least go to sleep for a while. But these only happen in the middle of the night for me and I have yet to find out why they happen.
And they're terrible. Also, not to mention, quite embarassing at a friend's house.