My little girl is fixed & has been shivering alot lately too. I really think its just the cold because when we put a sweater on her & wrap her up she stops. I did call the vet & he said just to watch her & if she has no other symptoms then she is probably just cold. She is still very active no vomiting or diaherra so I suspect she is just simply cold. But again I'm no expert or anything I just posted to because of the similarity.
