Pills!!

I’ve been on bc for a while now and I had a very very mild uti and was put on bactrim and Septra for 3 days and I had to take it twice a day. I called my pharmacist and she said I only had to use a back up birth control like condoms only for the days that I was on the antibiotics and that seems like that’s false. I waited 10 days after I was finished with the antibiotics course to have unprotected sex. Do you think I’m fine or should I be worried?