What Does White Pinkish Vaginal Discharge With Delayed Periods Indicate?

I was suppose to start my menstrual cycle last Thursday and I hadn t got it yet. I took a pregnancy test yesterday, 7 days after my missed cycle and it was negative. Well today, day after taking the test, in the evening im having slight lower back pain and when I went to the restroom after wiping there was light pinkish color to my discharge. Any Idea what this may be and why?