Before I read your question, once I saw 'regression' I thought "I wonder if there's a newborn".
Older siblings often regress when a new baby comes. Even if she loves her more than anything in the world. It's a big change - the routine is changed - and lots of other things.
You may try eliminating dairy for about a week and see if that helps. Otherwise, do some "play therapy" with her and have some stories with her dolls about new babies and see if she can play through some of her feelings. Think of this as her imagination working through what to do, so don't take any anger or mean actions seriously but rather let whatever she does get out and thoroughly hashed out. Try some different scenarios and see if you can find a starting point for some fear, anger, resentment, or whatever. Often once its played out, it goes away.
A great book is "Siblings without Rivalries". Which helps with accepting a new family member as well as their interactions.
Also - tell her lots of stories about when she was a baby and all the times you loved her and thought she was so cute. Let her know she had her baby time and baby attention as well as this new little one.