Is this a specific type of depression?

Usually I'm pretty optimistic. I mean, not excited or anything, just a regular gl half full type of person. But when I turned 26 I fell into a deep, deep depression where I was unable to get out of bed for over a year. I literally spent entire months in bed during this time as I did not have to work. When I was 28 I mostly got out of it but just as mysteriously as it came it went away. Now adays I spend most of my time feeling fine but every once in a while a deep depression will hit me and it comes out of no where! When it happens I just stay in bed until it goes away. People around me just think that I have a sleeping problem that hits me for one week every 2 months. They don't know what's really going on. I don't know of any kind of depression that just hits you for no reason every once in a while and then just goes away for no reason again.