My ex boyfriend had a similar problem. He went to a couple doctors and it turns out there was a build up of oil behind his belly button from all the unhealthy, greasy, fattening foods he ate in the past. He wasn't fat or anything, but he just ate really greasy foods for a long time. The doctors couldn't figure out what was wrong, and he was fine, just like you. He'd press on his belly and it'd hurt right on the belly button area and it started getting red. One day a marble sized ball popped out of his belly button, and it was greasy and smelled very bad...like bad cheese. Afterwards, he felt loads better and the belly button healed right up. That was the problem, and it fixed itself. I'm not saying that's what's wrong with you for sure, but it's a possibility. Check with your doctor and maybe he can do a scan.