Patti, sorry your little one was sick. Hope she is back to her old self soon. I found this and think it will help:
Blood urea nitrogen (BUN) indicates kidney function. An increased blood level is called azotemia and may be caused by kidney, liver, heart disease, urethral obstruction, shock, dehydration, or an intestinal or stomach foreign body. A low BUN may indicate a cirrhotic liver or portal shunt.
http://www.cah.com/dr_library/labtests.html