Took a treponema pallidium antibody test (electrochemiluminescence assay) 5 days after a course of phenoxymethylpenicillin (250mg tabs, 8 per day for a week for sinusitis). could this cause a false neg test? test taken 6 weeks post potential exposure?

Antibiotic treatment at five weeks after exposure would not have affected the treponemal antibody test had you been infected 5 weeks earlier. To remove any doubt, you may repeat the test in 3-4 weeks. for info: https://www.e7health.com/post/155/std-test-window-when-should-i-get-tested/#:~:text=What%20is%20the%20Syphilis%20Test,Syphilis%20is%20highly%20contagious.