Hi Dandaman, welcome.
When I started reading your story, I was thinking EBV, but you say that was tested. False negatives for Lyme disease are common; I'm not sure about false negatives for EBV. It does sound more like a virus though.
Were you tested for
CRP, and did that show anything?
In the weeks before all this started, did you have any vaccines? They can sometimes have impurities that can cause weird virus-like symptoms. These usually resolve in time, but it can take a while.
Any travel?
Doc