Antibiotics and birth control

My boyfriend and I are going crazy and probably psyching ourselves out but there's a chance we could be pregnant when we are not ready to be. We are using the pill and I take it religiously but I had to start taking an antibiotic, bactrim for a UTI that was causing me to pee blood. I took the antibiotic for three days and during this time we had sex twice and we're not careful, no protection was used. I took a test tonight and it tested negative but tests are meant for 4 days before my missed period which should be in approximately 17 days. Looking for words of wisdom and comfort 🙏