That feeling, or any feeling like it, it believed to be the subconscious building neuro pathways and learning how to treat information in these vast amounts differently. Your brain probably wasn't used to that, so it had to adapt, and in doing so it created that sensation.
Have you ever noticed how when learning something really new and complex in school, you might have a difficult time with it...but then after you've finally gone through all these weird mistakes, it suddenly comes together and your head feels like it's exhausted? PhotoReading is sort of like this at times, but the difference is your brain is handling it in a relaxed, effecient manner.
If you ever feel light headed because of it, just grab a bottle of water, and chill out for a few minutes.
The first time I PhotoRead a book, I had a similar sensation, and it was very intense. I still remember that first one right after the PhotoReading book itself...Decoding Your Dreams by Robert Langs. That thing was a good, relaxing read. I should go out and get that book...
Anywho, just my input. 
