I believe most people will test positive for Syphillis (syphilis) by 6 weeks so you might be to early, however syphillis (syphilis) is VERY RARE in heterosexual women.
Your HIV tests would be considered conclusive as the last one was taken at 19 weeks - you are HIV negative.
I think syphilis can take 10-90 days to show, but you should have noticed a syphilitic chancre, looking like a sore but painless, at some point. Other psoters may know more.