Also, are you sure you are on trimethoprim alone? Most of the time that drug is used in combination with a
sulfa drug called sulfamethoxazole; the most common trade names for the combination of trimethoprim/sulfa are Septra or Bactrim. If you took that combination, then the drug will not only interfere with testing, but likely would cure gonorrhea or chlamydia. So if your STD tests are negative now, it still might be possible you were infected earlier.