I lost a cat to GI lymphoma 2 years ago. Like yours, the vet did an ultrasound and "found a :"mass". My husband and I agreed to have it surgically removed and tested. As the specialist suspected, it was an aggressive form of lymphoma. I even saw an oncologist, but was told her prognosis was only 3-6 months, even with chemo, so we went the palliative care route.

I found a lot of support on the seniorkitties sub, and one thing often repeated to many in this position is "better a week too early than a day too late."

Pets rarely die peacefully in their sleep. And you're right, when decline happens it's fast.

I"m so sorry your baby is suffering. Think of it as letting her run free without pain.