Why do I feel so EXHAUSTED lately?

Should I go to the doctor? Do I need to just rest? Am I not eating right? Maybe I just need more excercise. And to quit smoking. Maybe I should be taking vitamins. I don't know. I just know that I feel like absolute sh*t. No really... it's true. Could it be depression? Is my Zoloft no longer working? I can't write anymore... I'm too tired.