Have you experienced fast mood shifts before? If this is the only time that this has happened, you may just be coming out of the depression. Sometimes depressive episodes just end suddenly.

You could have something called "cyclothymic temperament" (different from cyclothymia as an illness), in which you have daily mood shifts on top of your episodes of mood disorder. I have bipolar II with a cyclothymic temperament, and my moods are everywhere all the time. I have periods where I'm on average very depressed, but I may have a few hours during months of darkness when I actually feel hypomanic.

If you have a cyclothymic temperament, odds are that you're bipolar, as cyclothymia is considered a "non-manic marker of bipolar disorder". However, you still could be suffering from "just" MDD.