What Causes Delayed Periods With Light Spotting And Cramps?

I had my tubes tied 3 1/2 years ago (pinched but NOT cut) and my periods have been very regular for the past 3 yrs every single month. I am about 10 days late (was due on the 13th) but have some very light spotting that has started today. I took a pregnancy test 3 days ago that was negative. I've been having some slight cramping for about 2 weeks. Is there a chance I'm pregnant or have anything to worry about?