It sounds like you might have had oral herpes, and if the
feverAllergic rhinitis
Coccidioidomycosis
Febrile seizures
Fever
Fever blister
Fever blisters and canker sores
Herpes labialis (oral herpes simplex)
Histoplasmosis
Malaria
Rheumatic fever
Scarlet fever was due to the same thing, it might have been your initial infection with HSV-1. However, you didn't catch herpes from the exposure you describe; herpes lesions cannot appear in less than 24 hours after exposure. Kissing can transmit syphilis, although the risk is much lower than with genital exposure. Syphilis symptoms take at least 2-3 weeks to show up. It also is true that the standard syphilis blood tests typically take up to 6 weeks to become positive--so your negative result at 4 weeks is somewhat reassuring but not definitive. If you know the man and/or can locate him, give him a call and ask whether he might have oral herpes or syphilis.
So to answer your questions:
1) The chance of catching syphilis from that exposure is low, but perhaps not zero, if your kisser happened to be infected.
2) French kissing is a good way to transmit or catch HSV-1, but your symptoms came on too soon.
3) The negative syphilis test indicates you probably didn't acquire syphilis during the kissing episode, but to be 100% sure you would need to be retested again at 6-8 weeks.
Best wishes-- HHH, MD
HHH, MD