Light-headedness is either lack of water or too much water. So sip water gradually throughout the day, and make sure you're getting enough, but don't gulp it down in large quantities all in one go because that can make you feel under the weather and mess around with your sodium levels and your electrolytes.
It is quite common though, and usually not anything to worry about as it goes with the right quantity of water and a bit of a rest.
Don't forget when you get up out of a chair to do it slowly, that can also make you feel light headed.