What cause lightheadedness after drinking a cup of water?

May be nothing: The lightheadedness may not be related to the water at all but a manifestation of your underlying anxiety but if the water was very, very cold you could have had an excessive vagal response to it

Vasovagal episode: Ice water can cause a vasovagal episode, which is an episode of feeling faint due to a nerve response. To avoid this, one can try drinking room-temperature or warm beverages to see if it makes a difference. If not, the lightheadedness may be due to something else.

Lightheaded: It depends. Is the water ice cold? You may be having a "vagal" reaction to sudden core temperature drop when you drink cold water. The same thing happens to some people in cold temperature. If this is a vasovagal reaction, frequently you will also experience runny nose and watery eyes. Also, does you symptom occur when you throw your head back (vestibular) or even with sipping through a straw?

Just a thought..: Perhaps drinking it quickly and gulping in/down a lot of air along with each sip of water could cause this effect. Think about a similar situation when one is trying to control hiccups by repeated, stacked deep breaths- kind of the same idea which may be responsible for this reaction.