fatigue - physical and mental

Are you anemic (low blood iron)? That can cause a lot of fatigue.

Certainly get the D, B12, and thyroid checked.

Sometimes we can get 2nd immune conditions, so looking at Lupus is always a possibility as it is characterized by fatigue.

All that said, a number of us struggle with ongoing fatigue in a remission with no known cause. Just another wonderful extra of this awful illness.

This is a rather late response, I am a new member to this forum. To my understanding physical & mental health should not be distinguished. After all our body is a whole, when the gut is unhealthy that reflects on the mind and vice versa. I have periods of mild depression and anxiety, that only appeared after quitting smoking and having flares. There is ongoing research that suggests inflammatory proteins called cytokines, cross the blood brain barrier and are linked to feelings of depression and anxiety.
This is consistent with my experience, in the sense I had no clue whatsoever where these feelings where originating, my life was great and I had no reason to feel crapty and have negative thoughts. So I guess the struggle is to maintain our bodies in remission, if possible. I know it is really frustrating, but this knowledge brought me some comfort because I would tend to blame myself for my mental state.
Here is an article on the subject:
https://elemental.medium.com/the-link-between-inflammation-and-depression-a6e0d5c98639

I had a period of time when I was feeling exhausted all the time for no apparent reasons and what helped me a lot was fish oil