musikgal...I was thinking the same thing. If you're already taking supplements and still feeling weak, not eating enough or frequently enough could cause those symptoms.
A gallon of water a day is a little over the top unless you live in a particularly hot or dry climate. Generally, 8-10 eight oz. glasses per day would be enough (a little over 2 liters).
I agree with everyone else, though. If you're eating enough (at least 1,250 calories per day) and taking supplements and still feeling that bad, get thee to a doctor.