What Causes Missed Period With Cramps And Green Stool?

I am about 6 days late for my period (have taken two pregnancy tests, both showing negative), and for about the past 3 days I've had softish green stool. For the first couple of days of my missed period I was experiencing cramps as if I were about to start my it, but these past 2 days the cramps have completely subsided. I'm 30, went off the pill about 7 months ago, and have been regular with periods within 1 day (ie 26th or 27th). What is going on?