I've seen that in a buck that I got from another fancier. Luckily for me I had kept him and the other buck he came with separate from my other mice. Anyway, one or two days after getting him home he started bleeding as described (obviously from his bottom, being a boy) and was clearly very unwell. As far as I am aware it is what fanciers call scours. I'd guess in a doe it might be endometriosis (if I have spelt that right), which also involves reduced fertility.