OK, then your doctor is headed in the right direction. Infection of some type within your system is the reason for the infectious disease specialist and the pet scan........well not to scare you is normally for cancer or can also be used for further in-depth analysis.
I hit on the Epstein Barr as it seems to be some of your symptoms without regard to the being tired.
If he suggest one or the other I would try to get him to go with both.
With all the work that has been done and nothing showing up irregular there is little else to do or say.
I believe though you have some form of infection due to the fever and glands.
Sorry to not be more specific to your question.
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