My chihuahua has a bowel problem, I think? He only poops about every two to three days and when he pooped today, it was very dark and runny like. He doesn’t eat regular dog food, he is very finicky, I ususally feed him hamburger or chicken. Is the dark stool a sign of a problem? He doesn't seem like he is in any pain, he is active and happy, I am just worried about his bowel movements.