Antibiotics and birth control

At the beginning of last week got prescribed antibiotics and they didn’t mention much about it affecting my birth control. However just from reading I knew it could affect my bc so me and my boyfriend used condoms as a back up. This week I was supposed to get my withdrawal bleed since I started my placebo week. Instead of my normal period I only spotted for a day, it was dark brown blood. Could I be pregnant? This have never happened and I’m worried.