Starting antibiotics after unprotected sex while on birth control

Patient : Starting antibiotics after having unprotected sex on birth control, could I be pregnant? So I have been on Ortho Tri Cyclen Lo for about three years now. Haven't had any problems it's been really great and has helped me keep regular periods. I'm slightly concerned because I let my boyfriend ejaculate inside me on 6/13 and 6/20. He usually finishes with a condom on, but things have been very intimate this month. As of today 6/25 I learned that I had a UTI and my doctor put me on an antibiotic called Nitrofurantoin and I've read that it affects birth control. Is there a high chance that I could be pregnant because of the unprotected sex and the antibiotics? I've heard that you're not supposed to have sex while on the antibiotic because it decreases the effectiveness of the pill, but does it make a difference that I started the antibiotics after having sex? I'm 4 days away from starting my green placebo pills and I know that sperm can live inside the vagina for 3-5 days so I'm concerned that the antibiotics may reduce the effectiveness and then in turn increase my risk of pregnancy.
Doctor :   Thank you for your question. If you had unprotected sex after starting your antibiotics on 6/25, then there could be a p ossibility of getting pregnant based on Nitrofurantoin's affect on decreasing the efficacy of your birth control medication. If you are concerned about being pregnant, we advise you to see a doctor and have pregnancy testing conducted. If however, you only had unprotected sex before being placed on antibiotics then it is unlikely that you could have conceived. Thank you for choosing

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