This could be any one of several things, but if the bird is happy and healthy appearing otherwise, it may be sexual behavior. Does she crouch down, put her wings out slightly and then sort of shake rapidly, while panting in short, rapid breaths? When it stops, is she immediately back to normal, or is she lethargic and tired?
I would recommend a complete workup, including x-rays and bloodwork to be sure it is behavioral and not a medical disease.
Good Luck - if you need help finding an avian vet, PM me with your location and zip code and I will see who I know near you.