Drinking too much water is not the problem but drinking too much water in a short span of time is. If we drink water beyond the processing capabilities of our kidneys, then they are not able to eliminate the excess water. As a result, water level increases in our blood and sodium level goes down. This condition is known as hyponatremia or water intoxication.
Sodium is an essential electrolyte which balances fluids inside and outside our cells. The imbalance caused by water level going up and sodium level going down causes the fluids outside our cells to move inside. As a result, our cells begin to swell. This swelling can cause multiple health problems. Also, if swelling takes place in our brain cells, then we can get headaches or nausea. This condition can also prove to be fatal.