Can Stress Delay Periods?

I am 39 female. I have had very regular periods always, i am two weeks late. I was sterilised back in September,i have done a pregnancy test to which was negative. I rang the doctors the nurse said not to worry it was just my age. I take Ketamine and Nabilone for oain relief could any of these be the cause. The nurse suggested a few things like stress or rapid weight loss, i havent changed anything in my life but i am a bit worried