Why does my body feel totally exhausted about an hour or so after I go to the gym?

I've been going to the gym for 5 weeks or so and work out an hour a day, 5 days a week. I'm watching what I eat (I'm not under eating) and have lost 11 pounds so far, which I'm very excited about. I eat a protein bar in the morning for breakfast before I go to the gym, but everyday when I come home, my body feels like it's wearing out and I have to lay down for about an hour. Should I be eating something else with protein or something else in it after I work out so I don't feel this way? Is my body lacking something after I work out? I drank a 20 oz. bottle of Gatorade today after I came home to see if that would help, but I still had to lay down. I did an hour on the eliptical machine and burned 655 calories which I was really proud of and tomorrow I'll be doing both weights and cario. I make sure I have plenty of fluids in me throughout the day, so I don't know what's causing this.. Any suggestions? Answers from personal trainers or anyone who works out would be greatly appreciated. Thank you.