[Blarg]

Carbs tire lots of people out. You could be one of those. I am. Spaghetti, for instance, usually makes me very sleepy. Carbs are great for marathon runners, too, but weight lifting in a gym is pretty far from running a marathon.

After a work-out, your blood sugar could be low. Try eating a piece of fruit or two after your work-out. It will help rehydrate you, and get some relatively non-volatile fructose into your system. Fructose tends to stimulate the release of insulin much less than sucrose, regular sugar does. So you won't get the crashing/swingy feeling of your energy going up and down when eating it. To keep your sugar cycle in line, stay away from candy, including a lot of those crappy energy bars, after a work-out, as well as soda. Soda fills you with sugar and dehydrates you at the same time, a double whammy.

Get some water in your system, more than you think you need.

You might also try some Vitamin C powder or pills or chewables. They help regulate your sugar cycle and actually provide quite a boost in energy all by themselves.

Finally, maybe it sounds funny, but a little nap never hurt anyone. A hard work-out is definitely draining, and combined with anything less than a perfect sleep routine and low-stress lifestyle, is likely to make you feel tired. You can feel good and tired at the same time, there's no contradiction there.