Maybe its cause I haven't applied for builde ryet? jk. Its probably based on weather, current events, stress, etc. There's a lot of factors going into it so I wouldn't be too worried.
Mood swings happen sometimes, the body is a weird creature. There is probably nothing wrong with you, hope you'll recover for tommorrow.
I don't know, the body is the amazingest machine ever created like Eistein said I think, but even it has its flaws... Hope you will recover soon!