Can i get pregnant while taking yaz and also Erythromycin (antibiotic)?

I have been on Yaz for about 4 or 5 months now. I also take an antibiotic for acne. I had no problem with the old antibiotic, it didnt mess with the birth control. It was called Minocycline. I recently (last month) switched the medication for my face to Erythromycin, and this month something is not right. I have been done with my bc pills (active ones) for 4 days now and still no period. Every other month it has been like clockwork when it comes. I took a test last night and it was negative, but im not sure if it is too soon to detect pregnancy. But, im supposed to start my new pack of pills on sunday but im not sure what to do if my period doesnt come. Any advice would be greatly appreciated! Thanks!