Could be poor nutrition, could be poor base/endurance. Without a good base, you won't be able to 1) go for long efforts, and 2) do very many intervals. Aim for more steady state rides.
For nutrition, if you're working out, you'll need more carbs. Please don't be on the high protein kick- aim for a diet that's about 55% carbs, 25- 30% protein, and 10- 15% fat. And try to get your fat from unsaturated fat sources- stay away from the trans fat and the saturated fats.
Koffee