So if I understand correctly, you and fiance were intimate on the last day of your period? And your period lasted for 4 days?
If that is correct, that would be CD4 (Cycle day 4) and it is not very likely that you would ovulate around then (and therefore chance of being pregnant). Usually ovulation only occurs around CD14. However, I guess it's possible the two weeks of antibiotics could have messed up your cycle, and caused you to ovulate early? Maybe one of the ladies on here who is a little bit more well versed in fertility matters can comment?
If you are really worried, why not just buy a home pregnancy test? If you late enough to experience pregnancy symptoms, I'm sure the test should be able to detect.