Apparently s/he suspected a strep
throat, mononucleosis, or both -- based on his prescription
for an antibiotic (
for strep) and prednisolone (often used in mono). Or it could be just a garden variety respiratory virus, e.g. adenovirus. Your girlfriend probably caught it from you -- there simply is no reason to be suspicious it's HIV in either of you.
But most important, the HIV tests never lie, as long as testing is done more than a few weeks after the last possible exposure, i.e.