What do you mean her rear was soiled? Was it manure or clear mucous? If it is manure she may have, or have had scours. The diarrhea can irritate the vulva. If is a clear discharge like egg white she may be in heat. Yes, doelings can come into heat when they are that young. I am not proud of it, but I once had a doeling kid at eight months which meant she had gotten bred at three months. Her name was Yoletta and I named the baby Yoletta's Surprise. Yoletta was not the only one that was surprised, I can tell you!! I don't think it is anything you have to worry about.