Obviously it is something you need to seek a physician for. But think about your food intake. Food is fuel for the body and if you don't eat enough of the right kinds of food your body will tire sooner. Fast Food all the time wil quickly take a tole on your energy level. Google JUICING and check out its effects on your body's energy.
I'm sure that reason of your condition is bad nutrition. You can smile, but wrong food you eat makes you feel tired because huge energy which your organism wastes for its burning. So I recommend you to eat more fruits and vegetables and your previous mood will come back, believe me.