While you were sleeping, your body was digesting the rest of the food left in your system, & now there's a bunch of waste that your intestines are trying to push out. Once you go, does your stomach feel better? If not, you should get tested for any digestive ailments, such as Crohn's disease, celiac disease, & irritable bowel syndrome. Also, if you've changed your sleep position, that could contribute. I notice that if I've been sleeping in more of a sitting position, rather than laying down, I need the restroom immediately on rising.