What could this be?

[Babychicks2020]

I have a 2 year old chicken. Her name is goose. A few days ago we noticed she had no butt feathers so we brought her in to quarantine her and we noticed she has this bump on her bottom under her vent. And her poop is greenish yellow and white. They are feed grass, corn, layer feed, and calcium. They get mealworms for treats. What could be going on with our chicken.

 

[Wyorp Rock]

A large abdomen could be from fat, fluid or an indication of reproductive issues.

Consider getting a fecal float to see if she has worms and make sure her crop is emptying overnight.
If she's active, eating/drinking, laying eggs, then just monitor her.