Go to a doctor ASAP. Conventional medicine has its weaknesses and blindnesses, but there are some things it's really good at. The combo of
tiredness in the afternoon plus digestional distress sounds like a parasite or infection to me. If you go to a doctor and find out that it is, than you have a straight-forward course of action to take toward feeling better and having more energy. If it's not, than at least you know, and there might be something a doctor can see that we all (including you) are missing.
Many of the suggestions in the other thread are good, but if you've got a stomach bug, they'll only go so far....