I am having painful cramping in my lower abdomen and back. The pain will spread out all over and then will also localize to one spot for a while. Right now I only feel it in my left side and it is working it's way to the front. I was due to start my period on the 19th, but have not, so when I started feeling this way, I took a pregnancy test, but it is negative. It feels like menstral pain, but it will not go away. I have had it now for three to four days. What could cause this pain, and why am I not starting my period with this kind of pain?