I think it is totally normal and common for energy levels to cycle throughout the day, dieting or not. I could give you a dozen theories as to how and why you're NOW feeling this but they would all be guesses. Personally I wouldn't worry about it unless your energy levels continue to go down.
Since I cannot resist guessing I would say that the reginmental nature of the IP diet and your adherence to protocol and allowed your body to find a good rhythm and the afternoon is when you're typically a little less active.
How good is my guess?
