Abdominal Pain in a 7 month old

A 7 month old child presented to the Emergency department late one evening due to vomiting and lethargy. She had been vomiting for the last 24 hours; changing from a yellow colour, to green (bilious) then to dark brown colour. She was having wet nappies and her bowels were opening normally. There was no history of fever. The rest of the family were well. She had an unremarkable past medical history.

On examination she was pale, apyrexial, with sats of 99% in air, BP 105/76 (mean 90), Capillary refill time of 2-3 seconds. Respiratory rate was 50 and heart rate was 160. Her abdomen felt a little distended; and in the nappy some blood stained stools were noted.

An IV line was sited, as was an NG tube. 10ml/kg bolus of crystalloid was administered. A plain abdominal X-ray was obtained (attached).

What is the most likely diagnosis?