Without wishing to sound too obvious - dehydration? If you're dehydrated you should be very thirsty and you're also quite likely to feel dizzy. Try drinking several pints of pure water (not all at once mind, be sensible about it) and getting some rest.
Bear in mind that if you're dehydrated and/or have been out in the sun too much then it can take a fair bit of water and a good night's rest before you feel right again. Go with the obvious first, unless you know for sure that's not the cause.
