What Causes Delayed Periods?

I have a very regular month cycle every since I started my periods I have never skipped a month. My husband and I have been trying to get pregnant for about 7 months. My period is 3 days late which is very unusual and I have no symptoms as to my cycle going to start soon. I have taken 2 pregnancy test which have both came up negative. Should I wait a few days and then take another test or what should I do?