At the beginning of the summer I had a very similar problem....

Like most people in this thread already said, eating more helps...

I was having trouble getting through my workouts and analyzed my diet. Every Day I was only getting 1700-2000 Calories a day, with about 160 g protien. Being a 160 lbs male with 9% body fat, that was just above my Basal Metabolic Rate (The amount of energy u use if u just sit in bed all day doing nothing) and I was clearly doing much more than that.

I bumped up my calories to 2700-3200 a day (which rocked cuz i love to eat) and after a day or two of eating much more, i found i was getting more pumped at the gym rather than beaten up and run down.

Definitely try eating more, I can definitely see how the low food intake might be a problem. If you are curious about your BMR and other metabolic rates there are tons of useful calculators on the net (just make sure u are using the right one) and they can tell you how many calories u need with a +/-20% error....which will have to do for now.