Answer

I'm sorry to say this may mean the cancer is progressing and keeping her from getting enough oxygen.
If it is warm where you are then you might consider trying her in some air conditioning (even if you need to drive her around in the car a bit) this may help reduce her labored breathing to a slower rate.
It is possible she has a fluid build up putting pressure on her lungs. it is possibly she just can't cool herself enough with the lung capacity she has.
You may want to see an emergency vet tonight if she is in real distress.
This site may help you locate one if you are not aware of any in your area
http://www.vetlocator.com/hotline.php
or your vet's phone line may give you off hours aid info.
Sometimes the way the cancer changes the lungs makes the dog more disposed to getting pneumonia and sometimes antibiotics can help with that.
There are three pages of info on this disease here
http://www.petplace.com/dogs/primary-lung-tumors-in-dogs-lung-cancer-pulmonary-neoplasia/page1.aspx
It is possible your dog has turned the corner from having enough usable lung to where the only way to get enough oxygen would be with an oxygen mask or tent.
My sympathies on your situation - no time is good to have a dear dog ill like this.
Hope this helps you!

Expert:	NancyH
Pos. Feedback:	99.5 %
Accepts:
Answered:	7/16/2008

Dog Expert:Rescue, Train,Breed,Care

30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics