birth control and antibiotics

my question is, how long should i keep using a backup method like condoms after taking the antibiotics, although i was on my placebo pills the majority of the 7 days i was taking the antibiotics so i was on my period whether or not that was caused my my antibiotics not working i sill would have been bleeding. okay so i finished my antibiotics two days ago and i'm not on my period although i am spotting a tiny bit which is usually normal for me to spot a few days after my placebo pills, so i guess what i'm asking is could have my antibiotics caused my body to ovulate? and if so how long should i continue to use a back up method of birth control?

thank you so much for helping, and if you've experienced getting pregnant from antibiotics affecting birth control share your story:)