A thyroid condition, hormonal imbalance, or viral infection like mononucleosis can leave you feeling lower than a toad's belly. Some drugs—antihistamines and steroids like cortisone among them—can also make you feel depressed, so be sure to tell your doctor what you're taking.
Keep in mind that when a primary care doctor writes an Rx for an antidepressant, this is not usually her area of expertise. Psycho-pharmacologists, on the other hand, are highly specialized in prescribing psychiatric drugs and are a good choice if you've taken antidepressants without success or know you want to try medication. If not, consider making an appointment with an expert who practices cognitive behavioral therapy to see if that non-drug approach might work.