WHY AM I HAVING CRAMPS BUT NOT PREGNANT?

I am 28 years old and my husband had a vasectomy from a prior marriage. I started cramping 10 days before my period and on and off they have gotten worse. This has never happened before!!! I am not on birth control, and nothing as far as stress has changed in my life. I am still a week from my period but are having other symptoms of my period like canker sours ( common for me after I have started) but no period. I want to be prego but dont think it is possible with my husbands vasectomy. What could the symptoms be?