possible adverse reaction to food change

Question
Several days ago I changed my 4yr old chihuahua's food to a different type(fish based instead of chicken), but same brand. After eating the new food she threw up 3 times and had a little blood in her loose stool. I fed her a little rice to help calm her stomach and switched back to her old food. She seemed fine over the next couple days, then yesterday I noticed what looked like a rash on the rear part of her belly in addition to a little swelling. She is acting normal, playing w/ her toys, etc. There is some blood in her loose stool again and the swelling is still there. Could this still be from the food switch, or do you think it may be something else? Please give me any advice you can. (It has been 6 days since I first changed her food.)

Answer
Most dogs will react adversely to a sudden change in their food. Diarrhea and vomiting are not uncommon in that case. A rash on her belly would also indicate some sort of intolerance to the ingredients.

Another thing to consider is that most commercially available foods with the exception of one do not have a fixed formula. That means that from bag batch to batch the ingredients that make up the food are likely to change to get the desired levels of vitamins etc on the bag, but the quality goes right out the door.

This is why so many people have dogs that get sick suddenly from one bag of the same brand.

Is she still on her old food now or the new food? If she is that sensitive to food like that you might want to put her on Science Diet Sensitive skin diet. You would have to switch her slowly over a 7 day period but once you get her on the food she won't have these problems.This food is made with a fixed formula so there are no surprises ever in it and her diet stays consistent.
I have my daughters cat on the feline formula and the little bumps she had all went away.
I have been recommending Hill's Science Diet and fed it for over 30 yrs and have never had a client bring it back because the dog was sick from it or had a reaction. They have a 100% guarantee also which helps.
Give it a try and see how she is after three weeks. It takes that long for the changes to be apparent.
One more note. You could try some childs Benadryl and see if her rash goes away. That will let you know if it's an allergy too, but the food make keep it there.
Let me know how she does.