Hi Roseanne, I'm Dr. Deb. I'll do my best to help you today.
I'm sorry to hear about this current concern for Chiara.
Certainly older cats can lose weight as part of their aging process but they don't typically have the other signs you describe.
Diabetes is certainly one possible explanation for her signs..hunger in the face of weight loss, and increased thirst/urination...but Hyperthyroid Disease can also present this way. Not all cats will vomit with either one of these conditions but some will.
It's sometimes difficult to distinguish between these two conditions unless or until blood work is done.
I particularly like Hyperthyroid Disease when it's diagnosed in an older cat because we have many good treatment options...I consider it much better than Diabetes as diseases go.
Diet, oral medication, compounded drugs which can be made into tasty treats or gels that are rubbed into the ear
or even surgery are all viable options for a Hyperthyroid cat.
I wouldn't necessarily assume that her diabetes has returned. I'd have bloodwork done to evaluate her glucose level and her T4 and hope that the latter is elevated.
I hope this helps. Deb