American bulldog which is not eating and losing weight

Lots more info needed for proper responses:
How old is the dog?
Is the dog up to date on needles and worming?
Does the dog have medical issues?
Has his food been changed lately?
How much weight has he lost?
How long since he has eaten?
Is he drinking water?
What did the vet say when you first contacted them about this?

The more information you can give the better answers that can be given.

She is having a false pregancy and had her to vets on Monday and is on hormone liquid she is 14mths old and has lost a lot of weight I think her injections is due

She is having a false pregancy and had her to vets on Monday and is on hormone liquid she is 14mths old and has lost a lot of weight I think her injections is due

She is having a false pregancy and had her to vets on Monday and is on hormone liquid she is 14mths old and has lost a lot of weight I think her injections is due

A false pregnancy usually has the opposite effect, they will gain a lot of weight to mimic the extra intake as if they were carrying pups.

Unfortunately this isn't a question that will have a definitive answer. It could be something as simple as the 12 month hunger strike, a problem that a lot of owners struggle with when a dog will just refuse to eat and drop weight between the ages of 12 and 18 months (it is actually a lot more common than you would think) or it could be something more serious.

Has the vet ran blood tests to try and find out if there is an underlying problem?

There are lots of things that will make a dog go off its' feed.
By injection are you referring to some sort of treatment or vaccines?
If you have recently had her at the Vet then it would be a good idea to touch base with him/her and fill them about your concerns.
Also, any vomiting/diarrhea? Could she have gotten into something? Like the trash can or some type of cleaning products or chemical?
How is her attitude? Is she her normal self?
Lots of variables.
Another check by the vet may be in order.
Let us know how she does.