Nausea is a complex reaction to many different stimuli in the body. As you may guess, water is not a typical cause of nausea, because the stomach is very used to seeing it and it is not a toxin. Nausea after eating or drinking can sometimes be caused by the stomach being too full. If you are trying to force the water down into a full stomach, you should try not doing this. There are conditions in which the stomach has trouble emptying. If you fill you stomach with water and the water has a tough time getting into the small intestine, then it could make you nauseous.
On a side note, not all physicians recommend you drink 8 cups of water per day. Drinking too much water can be quite dangerous. I think you should plan on drinking a few cups of water per day, and the rest you should drink only if you feel thirsty. Of course there are conditions in which you should drink more water than average and conditions in which you should drink less. You should consult your doctor to be sure how much water is safe for you to drink. The best type of doctor to see for this issue is your primary care physician
such as a family doctor
or internal medicine doctor