I was on sulfamethoxazole (bactrim) for a uti. i took all of the meds. then waited 9 days and took an std/sti test. it was negative. could it have been a false negative due to the antibiotics?

If you had gonorrhea or chlamydia, the Bactrim might have cured the infection. In that case, the negative test was a true negative, not false: that is, you were not infected when tested. There is no test that can detect past gonorrhea or chlamydia after cure. No other STIs are affected one way or the other by Bactrim. (If you are at risk for STI, you should have been tested before UTi treatment.)