What Causes Delayed Periods Even After Taking I Pill?

Hi,

The condom broke during intercourse. Right after this happened my boyfriend and I went immediately to Walmart and purchased Next Choice. I took the medication as prescribed. I has pain in my breast for a period of time, but I have not seen my period for the second month now. I have done various pregnancy test. I have used different brands. The tests all indicate that I am not pregnant. What should I do?