What Causes Missed Period Despite Having Negative UPT?

Hello, As of thursday jan.23 I went to the doctor and I was put on 2 differnet anti-Infection meds. So all of a sudden im a day late on my cycle. I took a pregnancy test and it came back positve. Can u tell me y and the doctors did a urine and blood test and that came bk negative!!!!Could the meds have something to do wit miss cycle