What the heck was that??

Wow, I don't know what came over me the last few days but I am some relieved that it has passed. This really bothers me to think I can be so depressed and hopeless and completely down on myself then just come out of it. It happens now and then (too often) and usually lasts about 3 days... its horrible and I wish I knew what triggers it and how to prevent it... or even what it is that brings me out of it. I mean, I did get a better sleep last night but that cant' be the whole thing; its so severe. I feel like my mind has been possessed by something evil, then it leaves and I'm myself again. Its actually kind of scary but I try not to think about it... I have to enjoy the times when I feel good because it could be gone again tomorrow.