Fertile mucus does not always mean ovulation, especially when using birth control pills. Antibiotic therapy can affect the bacterial flora and the consistency of mucus, but studies have not shown that it reduces the effectiveness of contraception (except for rifamycins, e.g. rifampicin, rifabutin, rifaximin). It is worth knowing that despite this, the opinions of experts are divided and many believe that there are too few qualitative studies available to completely exclude the effect of antibiotics on the effectiveness of contraception. Therefore, it is often recommended to use an additional, barrier method of contraception (e.g. condoms) during antibiotic therapy and for 7 days after its completion.

You can read more about the effect of antibiotics on contraception here: https://www.gdziepolek.pl/artykuly/jakie-leki-oslabiaja-skutecznosc-antykoncepcji

I also attach a helpful thread: https://www.gdziepolek.pl/opinie-i-pytania/91188/jak-powinien-wygladac-sluz-podczas-stosowania-tabletek-antykoncep