What Causes Delay In Periods After Switching To Patch From Yaz?

I switched from yaz 4 wks ago. I am now on the patch and my period is three wks late. I have taken 2 home pregnancy tests and they have all been negative.This is the samething that happened to me when i was pregnant a year ago. I didnt find out I was pregnant til I was almost 3 mthns. I really would like to know whats going on I dont want to have a miscarriage again.