Antibiotics + late period?

I am late on my period, my last menstrual cycle was August 30 to September 6. I am normally not late like this, this is unusual for me.

I took a pregnancy test on October 29th and it was negative.

According to my phone app that I use to track my period, it averages my days in between, and it should have been on October 9th, but now it's November 8th.

I don't know if it has something to do with the fact I was sick and on antibiotics (Azithromycin) and Mucinex D the beginning of October, or stressing about finding a job, or my grandmother passed away at the end of September or gaining a few pounds. Or moving back home from college since May.

I took the pregnancy test again like she said and it was negative again. So I don't know if I'm doing it wrong or whatever. I've had sex four times since my last period, all protected with a condom. Not on the pill.