Why does being in the sun cause nausea, dizzyness, and headaches?

I was in the sun walking around town with my friend. I bought big bottles of water from the store before we went walking and drank a second bottle while resting but still, I became nauseated and got a very bad headache that lasted a couple of hours. It always happens after I'm out in the sun no matter how hydrated I keep myself. Is there any other way to prevent this? I'm in Texas so its about 100 degrees every day and I think we were outside for about 4 hours.