Have you, by any chance, not been playing guitar lately?
The brain actually grows when you start playing music, it adds brain cell connections to enable you to process all the stuff that needs to happen at the same time. If you stop playing music, that part of your brain gets zero exercise. Unused brain cells get cranky, like a pissed off senior citizen with nothing to do.
This can lead to apathy and loss of motivation. Been there. I make sure I play guitar AT LEAST three times a week for at least an hour. And normally on the weekend I'll get in about 4 to 6 hours. So, basically, playing the blues can prevent the blues. Go grab a guitar!!!!