Caisiemay, I know how we obsess and worry over these 1000 lb delicates!
My new addition, 12 yr old QH mare, also had two very minor nose bleeds since I've had her (about 4 months), in the same nostril. Didn't occur during a vigorous workout, especially at the trainers, but was happening while grazing, or at least that's when I saw it. My only conclusion is that she may have very thin skin inside the nostril and probably poked it with a stiff weed stem.
Will have my vet check her during her spring check-up, but I decided not to worry over it. Hasn't happened since, and when it did, was very little blood, did not last long and did not phase her in the least.
Keep a record of how often, when, what she was doing so you can give the vet some info when and if you do decide to go further with this. Like I said, in Misty's case, I feel it can wait until her spring check-up. If anything changes dramatically, of course I will call my vet and look deeper into it. Good luck.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".