Here's a great article about "Water Intoxication" http://chemistry.about.com/cs/5/f/blwaterintox.htm

Basically, it's not drinking too much, but it's drinking too much too quickly and it's pretty difficult to achieve. Most people won't do this and from what I've read the majority of reports of "water intoxication" comes from people who were on drugs (like ecstasy) and obsessively drank water i.e. they were not in the right state of mind anyway to control themselves.

More likely than not you're fine.

My mom had gone to one of those diet clinics allegedly run by a doctor and they give you shots everyday. The tell you to drink one oz of water for every lb of body weight (that is about 1 1/3 gallons for her and she is about 5' tall) She was feeling dizzy and called them and they said to just keep drinking more water, more water. She did and the symptoms got worse. She went to her regular doctor and upon testing her electrolytes were so depleted that she was a few numbers short of going into a coma.

That is just her experience but the regular physician said it was from too much water.

here's a good article that answers your question:

http://www.mercola.com/2002/jul/20/water.htm