If it is the abdomen and not the stomach area i would say its from the heat. if you've ever seen a dead animal on the side of the road over the course of a few days, they start to swell from the sun beating down on them all day.
At the bottom is the common phospholipid ,phosphatidylcholine.lipids are a group of naturally occurring molecules the include fats ,waxes ,sterols ,fat-soluble vitamins.
