excessively enlarged gizzard on 12 week old ISA

[Runt of the Flock]

i recently mixed my 12 week old chicks (ISA browns, buff orpingtons and americanas) with my adult hens/pekin ducks. Everything was going well, they came back into the coop at night, everyone was eating well until i noticed a few days ago one ISA brown was having trouble getting places that involved flying/jumping. I didn't think it was a big deal since they are still young and a bit small. But today i noticed she looked very unbalanced and i picked her up. It appears that her gizzard is enlarged and i just assumed she is just full but i took it in to show my dad and she puked?? and while carrying her i noticed her gizzard is the size of my fist!! Now I've never seen a full grown hen with such a large gizzard let alone a 12 week old... I haven't placed out a pan of grit like i usually do because i wasn't sure if that could be harmful to my ducks but my chickens have outside access to free ranging in my woods where there are many rocks, bugs, weeds and natural grit. None of my other birds seem to have any similar problems so I'm a bit perplexed. I'm hoping someone will know how to help me with this situation i haven't lost any of my babies yet and i don't want to start losing them now

 

[Eggcessive]

You are describing an enlarged crop I believe, instead of the gizzard. The crop is in the upper right chest, and it sounds like hers may be enlarged possibly due to the lack of grit. Grit is used in the gizzard to help grind the food. The food is swallowed into the crop, where it is stored before moving on the the gizzard and onto the intestines where it is digested.

When you pick up a chicken with an enlarged crop you can risk squeezing it and making the chicken throw up. It can also happen if you bend them down and forward. I would separate her in a dog crate with only water. In the morning feel of her crop to see if it is still enlarged, and note if it is firm, doughy, or puffy. Here is a picture of anatomy:
https://www.thehappychickencoop.com/wp-content/uploads/2018/04/Chicken-Digestive-System.png