I had 5 pregnancy test come up positive. I had an infection in my breast and began taking bactrim (sulfamethoxazole and trimethoprim). Now my test are negative, can the antibiotic caus?

HCG testing. I strongly suggest that you get a specific blood test for pregnancy HCG beta subunit, qualitative and quantitative exams.
Recommend restesting. The currently available home pregnancy test kits are highly accurate and easy to do, though they are not perfect. The Bactrim (sulfamethoxazole and trimethoprim) is not likely cause of the negative test after some already positive. Maybe the last kit was defective. It is possible/common to have miscarriage and so the repeat test maybe neg if some adequate time has passed. If you have more concern, please consult doc. Good luck.