I suddenly feel down. Watch
Don't really think Medicine is that advanced to explain sudden dips / peaks in mood yet.
There are lots of factors that we don't know that could potentially affect your mood, like the levels of light (hence Seasonal Affective Disorder), air pressure even.
What do you like doing? Perhaps talk to your friends / family? Venting / moaning usually helps!
That is a nice way to look at it, I have anxiety so it stops me from talking to people I know about it as I don't want to bother them. But I just try focus on other things, does that work for you?