Yeah, happens to me quite a lot. I'm not sure it's a snap of the fingers type thing, but many days I find myself with the get-up-and-go happy go lucky yay I feel fine attitude and then suddenly I've been crushed by such a small thing. It also only takes about half a second for my mood to change when I get forced into conversations with other people, too.
It's not so easy talking with my Dad though, since he views everything as negative and I carry no tone in my voice apparently, the only way people can test if I'm happy, sad or annoyed is by how quiet or loud I'm saying it, and the speed at which I say it. Anyway, sometimes Dad will snap at me because I'm being sarcastic or otherwise moody when I feel fine, and then instantly my mood becomes what he was accusing me of anyway. Which ruins the rest of the day.
And yeah there have been times when I've woken up, felt fine, felt I would do something and then somewhere along the line that line gets severed by something, and then it's nothing but a worthless boring day to look forward to, with nothing in it. There's never anything in my life..so once my mind remembers that I guess it just shuts down or something. No clue.
Yeah though, I feel your pain. Mood swings or not, whatever it is, it sucks. At least it's rarely an angry-violent thing for me, more just melancholic depression.
Also, a quick addendum, since I read you were deleting all your profile. I know once in the very bad past when my life was nothing but darkness, I got my girlfriend to systematically delete all the pictures of me, her conversation logs and generally any trace of me...it broke again when I tried "forcing" her to destroy all the gifts I had got her, at which point my other self (or something) kicked in and realised what I was doing. Then the standard depression rolled in to replace it, but that's better than hurting your loved one for no apparent reason other than a mood swing (or something.)