Have you ever had a day where you really felt down about life in general but weren't sure why you felt that way? You couldn't identify a situation or cause for the way you felt.
I've heard it said that our bodies retain memories from the past about which our minds may not be aware any longer. So sometimes it can happen that you feel down because the body remembers an event or situation that happened on that day long ago. Your conscious mind no longer remember it—although if you dig deeply enough in your memory, it may come back. But your body does and is grieving—or is simply remembering.
If that's ever happened to you, simply accept that your body has rhythms and memories of its own that can sometimes be quite different from those of the mind. Having a down day may be the body's way of grieving. Often, the feeling passes in a day or two.
That is a time when acceptance is important. The phrase "Go with the flow" also comes to mind. Don't fight the feeling. Simply be with it and honor the body's need to remember something your mind may not. We are extremely complex creatures, are we not? Things such as this sometimes make no sense to us. Just accept your complexities and don't ignore what's happening. It's OK. It's normal. You're OK.