Had sex on day 17 of cycle. on birth control & we used a condom that didn't break, but was also 1st day on antibiotics zithromax250. do i need plan b?

Low Risk of Preg.: There is a drug interaction of Zithromax and Oral contraceptives: http://www.rxlist.com/drug-interactions/azithromycin-oral-and-estradiol-norethindrone-acet-oral-interaction.htm but the article states "low risk of failure", esp. for 1st day of treatment. Also the condom, if properly used, will help a lot. Don't think Plan B is warranted, if OCs were taken correctly. In future, ask your GYN.