What would you call "periodic" depression?

I know that, in general, I have issues with depression (and possibly adult ADD).

However, most places I look tend to say you have to be “down” for a period of time before it can be diagnosed as depression.

I don’t get depressed for long periods of time (or if I do, that’s my natural state and/or I don’t notice it). On occasion, though, I have “bad days” during which it’s nearly impossible to get anything done, because of the sense of worthlessness and general malaise.

Is this something besides depression? (I wouldn’t be surprised if it’s hormonal or linked to hormones.)

Does anyone else experience this?