She seems to have some abdominal swelling but any searches on google give me very generalized descriptions and no images to work with, so I have no idea if what I'm dealing with is a bound egg, a parasite, tumor, genetic disorder, old age or something else completely. (Google is useless when it comes to looking up finch illnesses -very general descriptions and no pictures).

(her feathers are wet cause I gate her a bath).
The area immediately below the "T" of her rib cage shifts/rolls (i assume it's her gizzard?). Symptoms other than what you see in the image is very loose/liquid droppings. I've compared her chest/abdominal area with Fly, Katydid and Cricket and they're all normal (no weird abdominal shifting or discoloration).
She's a very relaxed bird, but she's not lethargic and can be quite active when she wants to be. She eats and drinks and takes baths all the time. She socializes with her cage mates (3 other finches and a house sparrow), chirps and does typical finch things. The only reason why I realized she might be sick is because I had to clean a glob of dried poop off her vent that she couldn't seem to reach.
Also, and I don't know if this is relevant or not, but Bee's missing feathers around her head/neck that don't seem to be growing back. She was like that when I bought her. She also keeps her feathers puffed up (which she's done since the day I got her).
I assume whatever's wrong with her isn't contagious because none of the other birds have gotten sick and, near as I can tell, Bee's always had this illness/oddness (I've had her for about 3 months).
Any help identifying what's wrong with Bee would be greatly appreciated, so that if any other finches I get show similar symptoms I'll know what it is and how to treat it.