depressed is not a synonym for sad
I think depressed people are rarely sad, actually; they (we) feel something different altogether.
When you're feeling down (which happens occasionally to depressed people, as well) do something you don't do every day. Take a walk just for the heck of it; buy something (in an actual store, not online) that you probably shouldn't buy or don't need; log out of FB and IMs, turn off your phones and watch 3 movies in a row (unless you do that all the time...).
You know, me-time. Be selfish. Being helpful, compassionate and productive is hard and takes a toll on your soul.
and stop making yourself feel worse by telling yourself you have a permanent clinical condition, when it's just a crappy day. No shame in having a crappy day. Most days are crappy, but we're often too busy to notice.