What Causes Delayed Periods With Lower Back Pain And Stomach Discomfort?

I usually have my periods on time I am now 4 days late. I took a few preg test and all negative. I have been have slight lower back and stomach discomfort along with more gas then normal. I thought it may be stress so I have taken the past two days to relax and be stress free but the discomfort is not going way. The discomfort is not bad at all just frustrating, since the test said I'm not pregnant what could this be?