Why is my period late??

I am 30 years old and my husband had a vasectomy 3 years ago which results in a zero sperm count 3 months post surgery. My period is 5 days late, I am ususally 28 days plus or minus 1-2 days at most. I am also currently on antibiotics for a UTI and am not sure if that would cause a late period. I took a First Response pregnancy test and it says I am not pregnant. I am not stressed out and not doing any new diet or exercise. Could there be something wrong or maybe on a far off chance pregnant?