Cornish with a large red bump behind left leg.

[RoostersAreAwesome]

Oh I forgot to mention, she is also loosing quite a bit of feathers when she preens herself and also in general.
The nest box she sleeps in has around 10 feathers every morning when I take her out. I can’t tell if they’re all hers because my other Cornish sometimes hops in her crate and sleeps with her.
Both sleep in a nest boxes because they are both too heavy to roost and my larger one got a bit raw from sleep on the concrete floor.

That’s probably because she’s molting.

 

[Snicketysnoksnaksnik]

That’s probably because she’s molting.

Could be, I never thought about that

 

[Snicketysnoksnaksnik]

Her bump is the exact same as yesterday except it feels more hard

 

[Eggcessive]

Can you feel an egg or anything with a finger when you insert it into the vent, to feel for a stuck egg? Unfortunately it is hard to know what the mass is for sure. We learn a lot after death when we can perform a home necropsy and look at the abdominal organs.

 

[Meyer_Hatchery]

It does sound like she's molting and from what I can see in the photos, her abdomen looks fairly typical for a heavy breed like a Cornish. Perhaps you notice it more because she's losing feathers?? The only other thing I can think of is to check closely for lice and/or mites that would be causing redness and feather loss.

 

[Snicketysnoksnaksnik]

Can you feel an egg or anything with a finger when you insert it into the vent, to feel for a stuck egg? Unfortunately it is hard to know what the mass is for sure. We learn a lot after death when we can perform a home necropsy and look at the abdominal organs.

Just put her away and tried, I can’t feel anything stuck

 

[Eggcessive]

I haven’t had meat birds before, so maybe it could be something normal. But it sounds strange being more to one side.

 

[Snicketysnoksnaksnik]

It does sound like she's molting and from what I can see in the photos, her abdomen looks fairly typical for a heavy breed like a Cornish. Perhaps you notice it more because she's losing feathers?? The only other thing I can think of is to check closely for lice and/or mites that would be causing redness and feather loss.

It could be that she was always like that, I thought that something was wrong because she is acting differently.

She doesn’t have any lice or mites on her bump (or around it). On her neck she does have mites (an issue all my chickens have, and some other birds that I check), can’t figure out how to get rid of the mites though.

 

[Eggcessive]

Mites can make them anemic and die, so I would get some permethrin garden and poultry dust at your feed store and dust them all over once and again in 7 days, and possibly in 14 days. Permethrin spray can be used to treat the coop, nests, and roosts after removing all bedding far away, and replacing it with new. Here is a link with pictures of the dust which you can find at feed stores.
https://the-chicken-chick.com/poultry-lice-and-mites-identification/

 

[Meyer_Hatchery]

She doesn’t have any lice or mites on her bump (or around it). On her neck she does have mites (an issue all my chickens have, and some other birds that I check), can’t figure out how to get rid of the mites though.

Definitely get on top of the mite issue. I know it can be difficult, but the mite issue can lead to trouble as @Eggcessive suggested. Here's a bit of info for you on dealing with mites.