J.N.
If it was just changing brands, I'd say you don't need to talk to the doctor. However, if you're changing from regular to something else, like soy or lactose free, I would definitely talk to the doctor first. You don't know what is causing the irritability and playing around could make things worse. It could be something as simple as the iron (my daughter had a really hard time with the iron, but did fine on "low iron" formula), or it could be lactose intollerance, difficult-to-digest proteins, or something totally unrelated to the formula.