Lots of times! I've been working out for 20 years now, and have gone through lots of highs and lows.
It could be any number of things....sometimes before my period, I seem to get real lazy and lethargic. That always lasts like a week, until I finally get the darn thing and feel back to normal.
On the other hand, it's definitely "bug" season where you could be fighting off a flu/illness and it's making your body work "overtime" just to keep it at bay.
On ANOTHER hand, you could be possibly overtraining...and it's your body screaming at you to slow down and rest.
I definitely agree with Kristina that if it lasts more than a month that you should see a doctor just to rule out stuff like anemia, chronic fatigue, anything like that.
As Cathe would say..."Just an opinion, take it or leave it!"