My wife is a lactating mother who generally doesn't get much sleep and she's always complaining of dizziness and pains in body. Whats the cause?

Consider depression. I agree that dehydration is the most common reason for symptoms like these in a nursing mother, but post-partum depression can also present with physical complaints. If pushingg fluids and eating well isn't helping then consider this as a possible cause.
Dehydration. Lactating women need to ensure they stay hydrated enough for their own body as well as to make all the healthy breastmilk for their babies. Breastfeeding women should be drinking every hour.. Keep a waterbottle by her side at all times! dehydration may cause dizziness, as well as leg cramps. These symptoms may also be from electrolyte imbalances, so i would recommend calling your OB as well.
Worrisome. I referred this to many types of physicians for an answer. In any case, you can call the office of the doctor who delivered her for more information. That is typically the place to start for individualized health information.
Hormone imbalance. Post-partum depression is usually caused from a hormone imbalance. See a physician who will evaluate the hormones and correct the balance if that is the problem.
Many things. Sometimes the low estrogen levels associated with nursing seem to aggravate the joint pains. I have found in some cases addi g low doses of Estrogens seem to help ( it also helps with postpartum depression). The problem may be a decreased milk production, however.
Thanx for added info. "the doctor that delivered her just tells her to eat more which is what i tell her and she does more but the pains don't go away. These days she's suffering from pains in the soles of her feet and her hands and the back of the rib cage." eating better and sleeping more are good. Having a helper in the home occasionally is beneficial so that the woman has an opportunity to relax and go see a doctor.
Sleep deprivation . Patient should take enough sleep. If she still complains of the same symptoms, she should be worked up for anemia, thyroid problems, etc.
Dizziness. Nonspecific dizziness is commonly related to hyperventilation. Dizziness that accompanies hyperventilation, anxiety, or depression often builds up gradually, waxes and wanes over a period of 20 minutes or longer, and gradually resolves. There may be no sensation of "air hunger" since these patients are hyperventilating only to a slight degree. This usually occurs in settings that are at least mild.