Drink plenty of water. You're probably a bit dehydrated and because the brain is working it's noticing the shortage of oxygen.
Also check that you are breathting deep and even. Some people develop shallow breathing and even hold their breath whilst photoreading. In which case just relax. Take nice even breaths and have a glass of water before you start photoreading.
Once you photoread for a while that will subside. Just remember to drink plenty of water because it helps to carry the oxygen to the brain.
Alex
