Agressive Bulldog

Question
I adopted a 5 year old spayed female English Bulldog from a household that had to give her up. We have a 3 year old chocolate male lab also in the household. They are beginning to get along. The problem is when sometimes she goes after him to warn him to get away, we scold her by sternly yelling NO! Two times we have yelled No and pointed our index finger at her and she has turned around and bit us. Once she actually broke skin. I don't know if she is doing this in self defense or if it is behavioral. Majority of the time she is very playful and friendly but this is very concerning to me. Is there a way I we can teach her not to do this. She is a great dog and I want to do my best to fix this.

Answer
You didn't mention how long you've had this dog and that could play into this behavior. This could be her way of establishing her position in the house. If she didn't live with other dogs before, this is what's happening.

This works with puppies and it might work with her. When she exhibits this aggression, turn her over on her back as fast as you can and hold her there for 5 or 10 seconds if possible and tell her "NO". Then let her up and ignore her for a little bit. You are establishing that you run the show in your house, not her.

I think this is something she will work out of with time.