This sounds like my experience with COVID (including now, four weeks in, though ti a lesser extent to what it was in previous weeks). So I'd guess whatever the reason it's what happens when you are sick, and the reason you are noticing it now is that you are more sick than you used to be)
My experience wouldn't be inconsistent with thermos blood volume theory though, if COVID made me dehydrated. I am often dehydrated generally so it seems likely
