Lack of sleep cause. Fatigue. No energy. Weakness. Mind not all there. No appetite. Headaches?

Yes. Lack of appropriate sleep will cause all the symptoms you mention. A few days dedicated to excluding all the worlds issues, pampering yourself into a better state of relaxation and trying to get 9-10 hours/ night for a while can be both confirmatory and therapeutic. If you have seen the doc and nothing treatable is causing the problem, get a sleep aid and use it.