Why do I tend to have abdominal pain and diarrhoea after gastrointestinal surgery?

Possible causes are.
Shortening of the total length of the gastrointestinal tract and reduction of the absorption area of the intestinal mucosa.
During the postoperative recovery period, there may be mucus and blood leaking from the intestinal mucosa, stimulating gastrointestinal peristalsis and resulting in a shorter residence time of the chyme in the digestive tract, etc.