No, an antibiotic like amoxicillin can't stop you from getting pregnant. Plan B could have but has to be taken within 72 hours to be effective so it is too late. But it seems you may not know exactly how pregnancy works. It's a lot to do with timing. You absolutely risked pregnancy and unprotected sex any tie during your cycle risks pregnancy. However, there are times in the cycle in which you are more fertile. so, day one of your period is cycle day one. Counting from there, most women have about a 28 day cycle and ovulate around day 14 or 15. Ovulation lasts only 24 to 48 hours max, but sperm can live up to a week leading up to that. That window of the week prior to ovulation and ovulation days is the most fertile time and when 98% of women get pregnant. I don't know where you are at in your cycle to help you know if it was a more fertile time for you. But do you know when your period is due? If so, you can count back. You can also take a pregnancy when your period is due. good luck and let us know how it turns out.
Amoxicillin can not stop you from getting pregnant, though it can interfere if your taking the pill and increase the likely hood of getting pregnant. If you have missed your period then a pregnancy test should show an accurate result but if your period isn't due yet wait unti it is due.