It may be a phase but you need to step in and literally step in between both dogs. I had a dog like her and what I would do is to body block the bully from the dog she was bullying to teach her that I "owned" that dog. If I sat down and she interfered with me petting another dog, I would put her in a down stay. Not harsh but consistent. That did work.
Forget about "Alpha bitch". True Alpha dogs are calm and confident and do not bully.
Give her plenty of exercise and crate time and your Bulldog some private time with you. Up her obedience kills as well. Can you try herding or treib bal?
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015