thanks for using health care magic
Antibiotics have not been associated with delayed periods.
Normal menses ranges from 21 to 35 days. It is important to note this and calculate how long your menses usually takes for the past cycles. It is normal to have small changes in the duration say a delay in 1 to 3 days.
However, it is important to do a pregnancy
test which you have done to be sure its not a pregnancy. However, you will have to redo the test in a week later.
Best regards and hope this helps you.