What Causes Delay In Periods?

My last period was April 10th and I'm pretty regular (30 day cycle). I am now a week late with only slight dark brown discharge. Two days ago, I had some red blood when I went to the bathroom once and then it was back to brown. I also have slight cramps today. I would be excited with the thought of maybe being pregnant, but had my tubes tied 12 years ago. Advice?