tk, I'd like to ask, have you seen a doctor? You could have depression--not the situational kind, but the chronic kind that can be treated with medication. It's something that might be worth looking into if you often feel like this.
Other than that, all I can say is that it's probably normal to feel tired from time to time (but not all the time). Have you been restricting your foods for long? You might need a multivitamin. And it wouldn't hurt to have your doctor check for anemia.
Jay