What Causes A Delayed Period With Negative Pregnancy Test?

I have missed my period by a week and a half. I have never missed a period before. I took a preg test 3 days after missed period and 7 days after missed period, and both were negative. My lower abdomen feels full and i have had a dull lower back ache and head ache pretty consistently until about yesterday. My breasts feel full but dont really hurt. Could it just be stress to cause the delay or not? I have been on the pill for years.