What causes the body to feel weak even when you eat and drink plenty of water?

Food allergies: Perhaps you are "reacting" to some of the foods you are eating. Try the following: for 1 week, try eliminating, entirely, 3 of the foods you most often eat. If you are addicted / allergic to any one of those 3 foods, you will probably feel horrid initially but then, once your body gets through its "withdrawal, " you may well feel much better. It's worth a shot! good luck!

Many things: Autoimmune disease, anemia, depression, lukemia...The point is see your doctor as soon as possible.