What Causes Blood In Stool Of A Baby?

Hello,i would appreciate to hear another openion about my baby s situation. My baby god some red blood in her stool. Her dr told me that she is allergic to the protein in my milk and to switch to formula. I am debating if i should do that. She didnt have any blood in her stool for 3 days than god some again yesterday and today, also today one of her stools didnt have any blood, other 3 did.. What could be the cause? Also before all this happened she go a vaccine 4days befor this happened.