Im so sorry that your little girl has being sick. I will have her in my thoughts and prayers for her to get better soon. I don't know what can be wrong with your little girl but I read that high levels of BUN can be due to food with high protein and also when the dog is dehydrated.
http://www.peteducation.com/article....&articleid=989 http://www.vetinfo4dogs.com/dbloodwork.html
I hope the above links help you. Please keep us posted.