What Causes Delay In Menstruation Other Than Pregnancy?

I am 4 months late on my period. The drs that i have been to told me that I was not pregnant but they also will not do a blood test. I am very worried about this because I have been trying to get pregnant. I have had irregualr periods before but I have never missed more than 2 at a time. Now i am close to missing number 5 and I am very worried. What can cause this?