I ended up being diagnosed with cancer, and after a bout of chemoradiation, it disappeared completely (as far as scans can tell). Let me tell you, the difference in energy was nothing short of amazing.
I'm really not trying to frighten you, but I was lucky that it wasn't aggressive and got caught relatively early. Chances are it's not, but it could just as easily be another medical condition and it's worth seeing a doctor. There might be indications of things in bloodwork or other measurements, so be patient but firm with your doctor if they tell you to just "forget about it".