I'm sorry you had this happen today. Bulldogs, as much as we all love them, have enough problems as it is!
Tomorrow, you might want to contact your vet and ask them about a "mismate" shot that can be given to your female dog. While it is not my personal favorite thing to use, in some cases, the benefits outweigh the possible risks. Please discuss with your vet the risks, as they can cause a life threatening condition called pyometra that can affect your dog's future reproductive health.
The biggest health problem would be that if she is pregnant, she will be using her body reserves not only to grow herself, but grow her puppies. Good nutrition is going to be extremely important for the female.
Because English bullies usually need to have a c-section anyway for giving birth, I would assume that if you and your vet decide to let a possible pregnancy continue, that a c-section is going to be the plan. Even if she comes from a line of Bullies who have had natural births, the smaller diameter of your dog's pelvis because of her young age would make me plan on a definate c-section. Definately write down today's date as the breeding date.
My experience is that first heat cycle pregnancies are usually smaller in numbers of puppies, but this can be bad in another way because then those puppies actually grow bigger and become more problematic.
Good luck with your Bullies!
Dr. Tanis!
Dog Veterinarian
10 years experience in small animal emergency medicine