Dog 1 and 1/2 year old bloody liquid stool ?

My dog (pitbull terrier) has all her shots and was even given a parvo booster shot a little over 2 weeks ago. She was around puppy's who died of parvo after which she was taken to the vet checked on and was updated on her vaccinations. Last night around 5 am she started having normal diarrhea this morning her first stool was basically completely liquid it was blood colored and smelled like blood. She vomited this morning 3 times followed by her first bloody diarrhea at which point I took her immediately to the vet. She was given a parvo and blood test both of which came back as negative. I have all kinds of medications antibiotics to give her. But I and the vet are baffled what is this? Has any1 experienced something like this? I also have a male (bully pitbull) who is one year who is showing no signs at all the are separated and he is in the Kennel until I finish rebleaching the yard and house I've already spent 500$ and I have no real answers on what this is! I just want my baby to be ok both of them! Any information would be great!