Does your vet deal with many large animals? It sounds to me like she has had mastitus or possibly still has a low grade infection? The udder can go down and back to near normal, sometimes you can feel lumps in the bag, although not always. The scaring is a real problem and most of the time after mastitus that has been missed the quater is blocked. I do the same as Twizzel, any ewes that have it get culled.