What Causes Delayed Periods With Negative UPT While On Loestrin?

I have been taking loestrin 24 for almost a year and usually only spot during the 4 days of brown pills. At the end of my last pack of pills I took antibiotics for 4 days (during the brown pills). I did not spot at all this month. I took a pregnancy test this morning and it was negative but I guess I am still a little nervous. Any suggestions?