i have this theory. Basically, all of the medical theories can be easily scrapped and replaced with one simple and all-explaining one. It all revolves around cave trolls of different sizes and shapes running inside the human body, totally unaware of the damege they do. here are some examples....

headache: a couple of cave trolls having a break dance session in your head

migraine: cave trolls having a rave in youre head. depending on severily add alcohol, hard drugs and glow sticks

hangover: alcohol gets to cave trolls a few hours after you had it, therefore about the time when you need to go to work they are just starting to party. Another explanation may be having been picked up by your ankles by a pissed off cave troll while youre asleep and being used to demolish a small medieval castle

indigestion: cave trolls skateboarding in your gastro-intestinal tract

flatulence: when a cave troll dies they just drag him out towards the nearest opening and leave him there for a while

pain in your backside: Not sure. Probably teenage cave trolls