Usually if it's coming from one nostril only it's from the head or a cut inside the nostril. If it was coming out both nostrils it could be a sign that they were bleeding internally. If you worked her hard it could be exercise induced pulmonary hemorrhage.
If it's a "one of" incident, out of one nostril, I wouldn't be going into panic mode, but if it happens again then it needs to be looked at. (this is just me, I don't call the vet for every little thing, you may feel better calling).
This has lots of good information: Understanding nosebleeds - Horse&Hound online