Bright red blood in stool of dog?

My dog had a lot of blood in her stools and I found bright red blood on the floor. I called the vet immediately and was told that I should take her to the nearest emergency clinic for animals. They checked her stool and took some blood samples. They told me that she was probably in apoplectic shock, caused by being stung or bitten by an insect. This caused an alergic reaction that caused her to either have problems in her throat or digestive tract. They put her on intravenous liquids and kept her overnight. I then took her to my regular vet who continued the liquids. She was there for three days. Not sure that is really what caused the problem since nothing I have read since then would suggest this. But not being a vet, I am not 100% certain that it wasn't from an allergic reaction.