So to add a medical perspective filtered through humour...
There are indeed parasites that could be growing in there that would give the exact symptoms. The two major options sadly are based in South America and Australia however it is possible one got imported and infected you, rather than getting in through the belly button though they would have been injested in your food, then burrowed through your gut lining before forming a protective layer around themselves in your abdominal cavity and growing there, somthimes they live half inside and half outside the gut and feed off your food, this can lead to a vitamin B deficiency just to warn you.
Just think, they could cut them out of you and you could keep them preserved in a jar
More boring is the Hernia option, trying to poke the part of the bowel back through the abdominal muscle yourself is possible but awkward, and only works if it is indeed a hernia. This option is also boring and easily solved either by massaging it back (medical massage) through or a simple surgery (you could still do some workouts but would need a few weeks off sports.) It is relatively common in people who work out a lot so is the most likely option, however it is normal and boring so let's move on...
Cancer! Sadly your symptoms don't match most of the common types but there are one or two rare types it could be which could involve surgery and multidirectional radiation therapy which is actually pretty cool and interesting (from my perspective since I can do it and from a technical perspective it is very advanced, clearly not from the patients perspective though). You also get to try being bald. But yeh cancer isn't too fun and your symptoms don't really match so let's move on to...
Giant Cysts! It could deff be a giant gooey cyst. You can even think of these as wierd giant internal spots all full of puss and goo. If they drain them, you can also often see the syringe full up with your own puss, it makes you suddenly crave milkshakes! All that lovely infection and yellows sludge mmmmm tasty looking. There is even the option of you accidently bursting it INSIDE your abdomen! Saving the best for last, you could stick a big hollow needle in your belly button and up into the cyst then use it like a belly button hose and squirt it out at people every time you squeeze your belly.
And there is your mini, humorous round up of your possible illness!
