Thanks for posting your query at DoctorSpring.com
I can understand your concern.
If your blood test is negative and if you have no physical symptoms then it would be best to confirm this with specific tests for syphilis like fta-abs. After this test can a decision be taken whether you need treatment or not.
The sore is the first symptom of syphilis and happens in early stages. Incubation period that is the time between exposure and the sore ids around 3 days to 3 months. The average interval being 21 days.
Please let me know if you have any queries.