Google syphilis, I am sure your sore looks nothing like it.
no chance with kissing, say for instance the person had a sypllis sore in there mouth, im not sure they had just worried that I now have this sore in mouth all fits with time of symptoims etc, worried and have to wait at least 6 weeks for blood test
From receiving oral sex you can get gonorrhea and syphilis, but it is low risk.