Abnormal Fatigue And Loss of Energy...

For the past several months I have experienced a constant feeling of fatigue and severe loss in energy. I am 19 years old and in the Navy therefore this change is extremely noticeable. I have physical standards that have always been easy for me to meet ( being able to run a mile and a half in a certain amount of time, doing push ups and situps). Working out is a part of my everyday life not to mention an important part of my job. When i first joined a year and a half ago the physical standards were very easy for me to meet. Now, just a year and a half later when running has consistantly become more difficult along with sit ups and push ups. I'm the same weight i was when i got out of boot camp when i was in the best shape. But its not even the change in my energy when i work out, its noticeable in my everyday life too. Going up or down stairs, walking up hills, carrying almost anything tires me out almost to the point where I become so tired that my body just hunches over. Also I'm constantly tired. as soon as i get off of work I am so tired I fall asleep. This doesnt seem normal to me or my family... at work people are wondering why I can no longer keep up at division runs... please give me some idea of what this might be stemming from? or is my body just falling apart at the old age of 19? lol