With associated dark coloured vomit, this is likely to be an upper
GI bleed. The causes could be infections, parasitic infestations,
GI inflammations, gastric/ GI ulcers, oesophageal varices, GI growths/ polyps, diverticulosis, fistulas, fissures, AV malformation, GI ischaemia and necrosis, initial symptom of necrotizing enterocolitis, colon cancer etc. If there are no other associated symptoms, all these reasons would have to be looked into, during an evaluation and ruled out.