When I feel okay, it's in my nature to kind of question how long this may last, probably because I know I cycle and I invariably land up and down within no given time frame.

It's probably just some kind of intrustive thought playing itself out at the back of your mind.

If there's no reason for it in your environment and there are no factors that you can think of at present that would cause a shift ...

Perhaps let the thought go for the time being ....