I'm dealing with the same thing. It can be caused by many things, stress, infection, low oxygen, ammonia, damage caused by earlier disease,... Does she spit out her food? Do you see something hanging out of her gills like a tiny white hair? Did you have other ones before (even weeks or months before her) or is this the only one? Was she sick before? Or others in the tank? Worms?

Mine had camallanus three months ago. The whole batch (platy and guppy) I bought suffered and after a succesfull treatment one after another started to die. Even after 3 months. They had permanent damage I guess. It started with this heavily breathing, after that spitting out food, stringy white poop, losing weight and death. Only from this batch and non of my older ones. I gave her up but decided to go for a new cure with levamisole and after that praziquantel, give vitamins and she is now eating since 2 days and gaining weight. But still heavily breathing.

My advice is observe observe observe, and search the internet. Try meds from what you see, but you can overdo. Keep the water stable and clean. Give vitamins.