In the presented situation, the probability of pregnancy is very low.

First of all, it's important to understand that its intensity and the length of withdrawal bleeding can vary. Sometimes it can be more scanty or shorter than a typical menstrual period. The fact that you are experiencing withdrawal bleeding at the same time as always is a good sign.

If you have taken birth control pills regularly, their effectiveness is very high. However, some antibiotics (especially broad-spectrum antibiotics) may reduce the effectiveness of hormonal contraception. I don't know what antibiotic you were taking, so it's difficult for me to give a clear answer. We wrote more about it here:

https://www.gdziepolek.pl/artykuly/jakie-leki-oslabiaja-skutecznosc-antykoncepcji

Please remember that the infection itself, or simply fatigue, can also affect withdrawal bleeding.

To dispel your doubts, you should take a pregnancy test, about 10 days after intercourse. In the future, consider using an additional method of contraception, such as condoms, when taking antibiotics.

If you have doubts about this situation, it is always worth consulting a doctor who can provide more personalized advice. If you are not able to make a face-to-face visit, I attach a link to the online teleconsultation:

https://www.gdziepolek.pl/telekonsultacja/z-ginekologiem

I encourage you to read additional materials that may be useful:

https://www.gdziepolek.pl/artykuly/test-ciazowy-jaki-wybrac-i-kiedy-zrobic