Hey SN, I've been experiencing this on and off for years. In my case it had everything to do with diet (I don't eat red meat, pork etc.) so I was definitely lacking in protein. I think the US FDA recommends 30 +/- grams of protein a day and I was getting 4 grams....with too many carbs and sugars.
I started to drink protein shakes every morning (20 grams) and have reduced the carb intake and switched to whole grains instead of foods made with enriched white flour.
I have 1000x more energy now, have a better outlook on life, and (in the opinion of friends) look a hell of a lot better. Plus I have put on a few pounds of muscle that I didn't have before (added benefit).
Get checked out but before anything else, maybe talk to a nutrionist. Before seeing her keep a log of what you're eating for a week before seeing them. They can then point out what you're missing in your diet. As you probably know, our North American diets are the pits when it comes to proper nutrition....