gastric indigestion
It sounds like the similar issue with my last hammy - Tooby and I have link my post above and there were some suggestions from other users.
I have attached two photos of Tooby, from the similar angle you took the picture for your reference, one is when we first found out the abnormal area and the other one was taken 2 days before her passes.
And same with Souffle, Tooby only last 4 weeks from it starts. If I were you, I would ask for a scan (although the last vet we visited, they said it is too hard to do x-ray on Tooby), it may find out the reason behind maybe instead of trying out different treatment but still doesn't know what she is suffering from like Tooby.
P.S. Tooby was a pet shop hammy too
Sorry for my poor english, I understand your feelings. Stay positive, wish you and your hammy all the best and recover very soon.