1. Eating food made of blood or animal offal may cause melena.

2. Upper gastrointestinal bleeding, especially gastric and duodenal bleeding, can lead to melena.

3. Some ingredients in protein powder may cause black stool.

If your body has no other discomfort symptoms, it is recommended to stop eating protein powder and observe for a few days to see if the stool returns to normal. If the situation does not improve, it is recommended to consult a doctor for further examination to determine the specific reasons and receive corresponding treatment.