Does she tend to cough at certain times of day or when she's doing certain things?
Have you noticed a reduction in her appetite or activity level?
There is no such thing as hyper-vigilance with your bulldog - you know her better than anyone else so if you suspect something is amiss, it well could be.
And not to be alarmist, but this reads like the exact same post I made before I realized Linus' heart was going out on him. I took him to the vet for a cough and wheezing. They looked at his upper respiratory tract and said it looked okay but maybe there was irritation or infection and gave me an antibiotic. Well, it never really got better completely.
Also, if there is a heart issue, they will need to do an x-ray or ultrasound to get a clear picture of what is going on. I was told Linus' heart and lungs sounded good as well.
All of this said, that doesn't mean what's going on with Petunia is heart failure - it could be seasonal allergies and irritation but if your gut is telling you it could be something more, express these concerns to your vet!