You know, I'm not quite sure what I should tell you, because while I've seen a lot of beginner's experiences in what their sensations are during, before, and after the PhotoReading step, this is something that I don't usually see.
Are you in a rush? If you're in any rush, or are PRing it with a purpose of getting through it fast so that you can meet a goal(and serious consequences will ensue should you not meet that goal), this could be causing this strange 'sense of panic.' Sometimes we may not even be in a conscious state of panic, rushing, anxiety or whatever. Sometimes it's something that's completely beyond our normal waking awareness. Being in this state can cause us to get tired, and sleepy fast, as has been shown by studies(Read Oxford Guide to the Mind, a very good psychology related book for this source.)
Try making an affirmation, "I am totally and completely relaxed in every way needed for the PhotoReading process." It's a truthful statement.
Often times, people will experience something along the lines of a heavy sensation, or a tingling in the back of their brain after the PhotoReading step. This usually means that they're feeling their brain adjust to handling information in a different way than it normally would, does, or did. I like to call this the 'Beginning PhotoReader Syndrome.'
In any case - be sure to drink plenty of water. Keeping your brain hydrated is very important.
Hope the Mother of the PRing Community(AlexK) or another can help you out well. Oh, and I call AlexK that with affection. 
-youngprer
