Did you work out too hard? For the first attempt, you should start out light...work your way up. And make sure your getting enough calories, you don't want to burn calories you can't afford, your body needs fuel!
I'm a diferent scenario, I'm on medifast which is very low cal...800-1000 calories...and I know the first time I had heavy activity (I played baseball with 7 year olds, it wasn't that heavy, and no more then i'm used to) I almost passed out from too much activity and not enough fuel..I was completely drained all day, I had zero energy. Same thing when I brought the twins I nanny out for a walk...about 3 days into medifast we took a 2 mile walk and it was literally ALL I could do to get them in their high chairs, fed, and into their cribs...then I DID collapse. Now my body has adjusted and I can run 2 miles on the treadmill, no problem, and exercising regularly has given me SO much more energy. But I've been on this diet for 2 months, and I started out super slowly!