I keep looking at your horse. She does have a line.
A heave line is generally caused by a horse having to work so hard to breathe that she develops the muscle. I have a hard time seeing the horse that you show performing having difficult breathing. It is not something that you can really diagnose from pictures. Heavey horses that I have seen were really sick. They couldn't perform. They were too thin. They coughed all the time.
What does your vet say?