Sounds to me like the beginning of a depressive episode.

For me depression creeps up slowly, over weeks, finding its way into all facets of my life (except for when I went off Lamictal last year when the depression came up in a matter of days, but that was different from my regular ups and downs).

Maybe it's being caused by the transition into fall.

Since you say you're stable it could just be a passing thing but maybe try to at least journal your mood every day so you can spot if a trend is happening.

Maybe journal your mood in the morning, how much you slept, and then journal your mood in the evening (could be a simple one-liner something like "ok, 8 hours, feeling bad" just to keep it as easy as possible).

Edit to add...maybe get your thyroid level checked to rule out the possibility that it's low thyroid which can cause depression.