What Causes Delayed Periods With Cramping?

Hi, I have never missed a period in my life (except for when I was pregnant with my daughter), nor is it ever late. Mind you about two cycles ago my period came a week early, which has never happened to me before.
My period is now 3 days late, and I am experiencing cramping as if I am on my period. I know I am not pregnant, I have not engaged in any sexual activity in over 5 months.
I am not stressed out, and there haven't been any major life changes lately, what could be the cause of my missed period, and the period like cramping?

K.G.