What Causes Delay In Periods?

Hi My dr prescribed Utrogesten to me in the beginning of February, I used them on day 15 to 25 as said and was also prescribed Fertomid 50mg to use on cycle day 3 - 7. My period was due yesterday 20th Feb but no show so far except for n light brown pinkish mucus discharge. I took a hpt this morning but it came out Negative. What could be wrong? When my period doesn't show this month when do I have to use the Fertomid then if there is no cycle day 3?