What Causes Delay In Periods?

Hi, i missed my period last month but on Day 20 i got some light bleeding for 3 days so i thought i was pregnant but home tests showed negative. I called the doctor and she prescribed lutenyl for 7 days, now it's the 3rd since i stopped it and no period yet. Any idea when i should get my period? what would be the cause of all this disorder knowing i always had a very regular period. I am now 35 and trying to conceive the 1st baby. Thanks a lot.