Welcome to the Forum.  I'll be glad to comment.  There are several possible explanations, none of which involve you having acquired syphilis again and have more to do with changes in the syphilis tests than anything else.  It would help to know what the name of your most recent syphilis tests were.

Syphilis testing is typically done as a two-step process in which persons have and initial, screening test and, if the screening test is positive, then go on to have a second test using an unrelated test for the purpose of confirmation.  Until the past few years, most syphilis screening tests were conducted using tests called an RPR or VDRL test.  More recently however, the testing process has changed so that now, the type of test used for screening is a "treponemal" test  and, if this test is positive, then the RPR or VDRL test is performed.  The problem with this change in sequence is that the treponemal tests stay positive for life in most people so that people who have had syphilis in the past will have positive tests and worry that they have reacquired infection.

Alternatively, you may have developed what is called a "false positive" syphilis test.  False positive syphilis test may occur for any number of reasons and are often transient. 

Finally, in some instances, persons who have had syphilis in the past have fluctuations in their antibody levels which can lead to occasional positive tests.

Either way, given the fact that your partner of the past 5 years has negative tests, I suspect that your positive test is either a result of changes in the syphilis testing procedure or a false positive result.  Either way, it is unlikely the you have re-acquired syphilis and you may not have needed the treatment you received. 

I apologize for the complexity of this response- it is a complex matter.  Please feel free to ask for clarification in your up to two follow-up questions.  In addition, you may want to discuss your testing history and past and present results with your own doctor or someone from the local health department for the purpose of clarification.  EWH