What Causes Delay In Periods And Black Stool Despite Having Negative UPT?

I had a tubal ligation during my last c-section 7 years ago at the age of 20. I am already 4 or 5 days late on my period and showing symptoms of pregnancy (ie changes in breastfeeding size, moodiness, weight gain, tenderness in my stomach, distaste for food I normally love and eat, and so forth). I just recently took a test and it said negative. And tonight I had a bowel movements almost black in color (no medication taken prior ) I am concerned as to what I should do. Please any information would be appreciated. Thank you