It does sound like overtraining to me. But I'd really need to know your workout schedule and your past training history and current goals before I could really say for sure. You could also be coming down with something. Or nutritionally deficient - if you're tired most likely B-vitamins.
High level - always alternate hard days/easy days. Now if you're doing 20 minutes on the elliptical or treadmill at a heart rate less than 140 every day you're most likely not overtraining. Also make sure to spend some time warming up and cooling down. You actually get more fit as your body repairs the damage you cause working out. Also make sure to get plenty of sleep. Make sure its dark, quiet, etc.
As far as nutrition goes you want to eat lots of multicolored foods (and no I dont mean lucky charms - although they're def one of my fav's). Fruits, veggies, meats, lots of different colors on a plate. I'd also recommend at least initially trying a multi. It wont do much damage, and you'll see results in a week or two if thats the problem.
If you have any more specific info let us know.