What Causes Delayed Menstrual Periods?

I am 36 yes old average weight. I take morphine for chronic pain. Normally my cycles are to the date and minute. Last Sept my husband and I started trying to get pregnant. Ever since we have been trying, my cycles are all over the calendar. My last cycle was on April 7 and now it is May 18 and I have not started. Could the morphine be messing with my dates? Or do Ito know what the cause is?