Thank you for your question.
There is a window period where tests for syphilis can be negative. It is possible that a previous encounter got you infected. Did you at any time have a painless ulcer on your genitals?
Next Steps
please see a doctor specialising in skin and DVD coz you will need screening for several other infections. they can recheck your syphilis status too. Sometimes tests for syphilis can cross react with other antigens i. e you can have a false positive test but it depends on the titres. please ask your doctor to confirm