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

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