Thank you all. Remy, your situation sounds very similar to mine.
Currently she is in the barn with a buddy, as her sore rump was cracking/not healing out in the sun and rain, so I decided to bring her in last weekend. As soon as her sore has gone I will put her back out with some sort of jacket - would be awful to be bald outside at this time of year.
Thank you for the information and reassurance - I was very worried when I suddenly found her with two bare sides this evening - even though she is very bright / eating.