Actually, I do not think a chicken has a part called "stomach." It has a crop and a proventriculus and a gizzard and...
But I said "stomach" to point out a similarity:
On people, we swallow food and it goes into the stomach.
On chickens, they swallow food and it goes into the crop.
From what you describe, I think the crop is probably fine.
It is normal for a chicken's crop to be a bit off-center. It's also normal for it to move around a little bit.
If you have other chickens, try feeling for the crop on each of them to get an idea of what is normal.
Crops are usually...