If you’re going to the vet anyway have them pull blood to check for Addison’s and Atypical Addison’s. My 4yo Boxer presented similarly, they diagnosed-ish Atypical Addison’s, but we ended up losing her. I still can’t tell whether it was the AA or whether she got tainted Diamond kibble, as the AA should have been very treatable and she just didn’t respond at all.
Hopefully it’s nothing major