Doesn't appear to be a cyst. I didn't see any viscous purulence (puss) normally associated with such a growth. I think you're also correct on it not being a tumor. Those normally are more dense and contain vascular bundles that would bleed out during a necropsy. The symptoms and visual presention lead me to believe the sen suffered a hernia of the peritoneum (sac that holds the body cavity organs in place). A hernia would cause the intestinal tract to prolapse outside the body cavity. This leads to insufficient blood flow to the intestine and decreased peristalsis which is needed to "move things along".
I'm sorry for your loss and applaud your efforts in investigating the cause of death for the benefit of other hobbyists.