Random Sadness: Is It Depression?
Random sadness/feelings of emptiness or worthlessness can occasionally attack you, and it can feel like a sudden onslaught of depressive feelings, as seen in this Reddit thread and this forum thread. Depending on the duration of this "wave of sadness," it can precipitate a depressive episode, or can just be a sudden sobering moment in an otherwise happy time.
I've experienced both kinds of random sadness--the kind that leads to a depressive episode, and the kind that flow in and out of my brain, going as easily as they came. If your "random sads" don't last more than a day and don't happen but every month or so, you're probably not experiencing depression--but keep track of when you feel suddenly and inexplicably sad/empty/worthless, and see if you start noticing these feelings coming on quicker and staying longer. This could signal the start of depression.