The skin on your neck is too thick for syphilis to infect, and kissing doesn't transmit syphilis.
Syphilis is a round, open sore that is painless. It doesn't "open" like you describe.
However, I really think you need to get this thing checked out. It could be a pimple, but could be an abscess or something that needs antibiotics.
I would be very surprised if it's syphilis or anything at all related to your kissing encounter. Go get this checked promptly and let us know what happens.