Big C,
I know the kind of days you mean. Where you're feeling kind of blocked and unmotivated, the day passes and you feel as though nothing has been accomplished. You may also feel unsociable and irritable.
I've been there many times and I have tried to suss it. I think it's a form of deppression and normal deppression is always a symptom of something rooted elsewhere. If you don't mind I'll make a few suggestions on what the root cause might be, based on my own personal observations.
Hangover from drink or drugs, late nights, overwork, unhappy in your work/relationship/friendships, unresolved business, anxiety/guilt from gambling/infidelity/dishonesty/obsessions etc, unfulfilled or unexpressed talents.
Try to learn what it is that gets you out of those moods or what it is that gets you interested and excited and force yourself to do those things, when all you feel is just like giving up.
Hope it's helped.