They can have enlarged lower bellies from fluid, internal laying or salpingitis, and more rarely, a hernia in the muscle wall, but the most likely answer is that they pecked her belly raw. It is a common place for agression. I would cut back on a lot of the treats which are diluting the protein and other nutrients in their feed. 16% is the minimum protein they need, and scratch and vegetables don’t have much protein at all. Scratch is 7% protein, lettuce and other fruits and veggies have little if all. Meal worms are good in small amounts, but I would get some Purina Flock raiser 20% or another brand, and make that 90% of what they eat. Be sure to keep some crushed oyster shell available for hard egg shells.
Keep her separate through a do crate or some fencing until her skin has healed. Keeping it coated with plain antibiotic ointment will help it to heal.
There are many good articles on how to prevent vent pecking, but I would spend some time looking for bullies. Too much light or too high of roosts are some possible causes. Here are 2 good articles to read:
https://extension.psu.edu/poultry-cannibalism-prevention-and-treatment
https://www.fwi.co.uk/livestock/health-welfare/causes-vent-pecking-cannibalism-layers