Two weeks from the last time you had sex you can test with first morning urine. As for if you should be concerned, anytime you have sex--regardless of protection or not--pregnancy is possible.
antibiotics pending on what type but most cancel out your BC leaving you with a greater risk of getting pregnant thats why your provider most of the time will tell you to use a second form of protection. i would take a pregnancy test its your best bet
If your miss your period then test. Antibiotics tend to make BC ineffective so your chances of getting pg are greater when on one. Always us a backup when you're on an antibiotic and until all the meds are out of your system.