Ditto to what others have said about the timespan. It's good to go see the doc however, because no matter what the cause, you're still having some troubles. Iron deficiency, eating disorders, viruses, stress, pregnancy, overexercising (just for starters) could also be a cause.
The ones I pity are the ones who never stick out their neck for something they believe, never know the taste of moral struggle, and never have the thrill of victory. - Jonathan Kozol