Personally, I haven’t experienced sudden changes in mood that are as dramatic and enduring as the one you describe. Although I have found myself suddenly feeling sad or low sometimes, I can usually pin-point a cause for this, or a thought that entered my head that caused me to feel in a negative way.
If you continue to feel troubled by your mood, or just “not yourself”, for much longer than a couple of weeks, I would advise you to speak to your doctor. Sometimes, certain mental health problems, such as depression, can be endogenous in the sense that there isn’t an obvious external cause for a person experiencing symptoms of depression. Don’t get me wrong, I wouldn’t dream of insinuating from what you have said above that I think you should be worried about your mental health, as I think it’s perfectly normal for moods to fluctuate from time to time, and for some people, that fluctuation will be stronger than for others. All I mean to say is that I would encourage you to be mindful of your own mood, and what feels “right” or “not right” for you, and go to your doctor if you continue to feel that your mood feels abnormal or strange to you in any way.
Unfortunately, in a number of cases, when people are ill as a result of a mental health problem that began with no obvious cause, they continue to go on feeling low without seeking help because they feel like they should just “snap out of it”, or because they haven’t had some sort of personal “crisis”, they shouldn’t or can’t be depressed. This isn’t the case at all, and your doctor should be able to help you, or direct you to other sources of help, for any sort of health problem you are experiencing.
I hope you start to feel better soon.