Possible causes are abdominal bleeding, organ rupture, or inflammation of the abdominal wall lining.
Lumps, Bumps, Swelling - any local swelling of any size on your cat
could be an abscess
but it could also be a tumor (benign or malignant), cyst, insect bite, hematoma, fracture, or soft tissue trauma (sprain, pocket of fluid such as infected oil gland). Vet care should immediate.