@dorma - I feel pretty much the same as you do. I’m very weak, unless I’m resting in bed for a while. Getting up going to bathroom makes me short of breath. I’m like this every day now since spring. Before that I was fatigued, but not like this.
I think I know why, because I had similar 10 years ago when I was finally diagnosed at Mayo with an autoimmune GI disease. I was treated for it and I was fine for 3 years- until onset of the pandemic. I have never had Covid, but I started having some of the GI symptoms again.
I have an excellent PCP who is helping me trying to see a good GI. I have an appointment in 2 months locally. Mayo rejected my request this time, but I asked them to review my request.
It is possible that you have some inflammation in your body that is not showing up on regular testing. It still means that something is wrong with you and your doctor has to consider everything. Keep asking!