Gastrointestinal bleeding may also be accompanied by: palpitations, dizziness, darkening of vision, cold sweats, fatigue, shortness of breath, and even syncope. These often indicate that the bleeding volume is already large and the body can no longer compensate. However, sometimes eating too much duck blood, pork liver, or certain iron supplements can also turn stool black, but it is usually not shiny or sticky and disappears after stopping the medication or food.