I am sorry you have waited so long for a reply! I have only just come on-site again.
You have to take her to a vet. It's not possible for me to tell you if this is a problem caused by Cushing's, or if her breathing problems are related to the breast lumps.
She may need at least an overnight stay, with oxygen therapy, and likely a chest X-ray. Also I can't say if she is fit enough for surgery, but an incisional biopsy of one of the lumps might be needed (if she is.)
However, that would depend on the results of the chest X-ray in my own opinion. So it's likely that will come first on the agenda.
A "needle biopsy" could possibly be done, which doesn't need an anaesthetic, and is done quickly (usually only takes 10-15 minutes) But whether the vet could give same day results I don't know, or if the samples would have to go away to a lab.
Also needle biopsies are not 100% accurate and can sometimes give false negatives. But it might give at least a clue as to what is going on with the lumps.
But do take her in. She is very uncomfortable and needs a proper assessment.