Sending a big hug because I too had a spell of isolation and doldrums. I think it happens to most of us once in a while, but that doesn't make it fun.
Your recovery is shining because you clearly recognize that something is off balance here. I agree with the posters above, if you have trouble shaking it after a few days, a trip to your doctors may be in order.
Lots of things can hit us when we're not looking, PTSD is common here, but also depression or thyroid problems can be helped with proper medical attention.
For me, the difference between a bad day and a good day is generally about 2 days. After that I need to take action to try and shake off the darkness.
Hugs