What are you feeding?
Specifically, are you feeding alfalfa or any other source of high protein feed?
A lot of horses will sweat more when eating alfalfa. They also drink a lot more and urinate a lot more on alfalfa.
As long as she is drinking enough to have sufficient water in her gut for digestion and defecation and does not impact from a dry gut, it is no problem.
I also used to ride endurance races and I have trail horses now that go 100 miles a week in the summer. Last summer they were being ridden that much in 100+ weather. They did not even sweat (except under their saddles) once they got used to the work. Sweat poured off of them and ran down their legs when the heavy work schedule first started. Within a month, they seldom broke a sweat. Their water consumption went up and down with their sweating level. Not one had a bad day and not one lost weight and 2 are over 20 and 5 or 6 are between 15 and 20 years old.
If you are really worried about health, have a Vet pull a blood sample and have him run a full blood chemistry panel. It will check for liver function, kidney function and several other things that indicate how a horse's body is working for them.