Hello~For your peace of mind, if you haven't already done so, I would again see your GP for another blood work up, you are probably fine, but it will give y0u added peace of mind.
Receiving fellatio has NOTHING to do with your current fever. Zero risk for HIV, and you have tested conclusively negative for everything else. Forget about this event as the cause of your fever.
There are literally hundreds of causes for a sustained fever, most commonly a viral or bacterial infection, some of which could be serious. See a doctor for a diagnosis, but it is certainly not related to your sexual event.