While heavy panting could mean heart problems, it could also mean she just has a tempeture.
My last boy died of a heart tumour and about the only outward sympton in the early days was his panting so when Alfie started to pant two weeks ago I was off to the vets like a shot. Thankfully he was just running a tempeture of unknown origin and a course of antibiotics had him right as rain
Do take her to the vets even if its just to ease your worries, I doubt its anything sinister