very light short period, pregnant, UTI

a week or so ago i had a Urinary tract infection and was put on anitbiotics, i am on birth control. right before the UTI i had sex, he puts a condom on before he climaxes but we dont use one at the beginning, and i know there is some seimen before climax. i got a uti, was put on anitbiotics and now i had a very short, light period. could i be pregnant or do anitbiotics have the potential to alter your period because i know they affect the effectivness of the birth control and i hadnt been taking it at the same time a few night in a row.