What Causes Delayed Menstruation While On Aygestin?

Hi, I am a 37 yr old female and have taken Aygestin 5mg 1x per day for 11 days for abnormal uterine bleeding. I began the Aygestin on Dec. 13th 2014 and continued through Dec. 23. 2014. I still have not gotten my period and pregnancy tests have been negative. I am trying to conceive and this is very confusing??