It's not uncommon and won't necessarily happen again.
The capillaries are very close to the surface and I have seen it happen after exercise, a bumped head, no apparent reason etc and have yet to know any horses where it happened more than once or twice or was at all an issue.
If it gets worse call the vet, if it starts happening regularly call the vet. Otherwise don't worry about it at all.
Doesn't even sound like she was really even bleeding, just pink tinged snot. Even if it does get worse I wouldn't worry too much about it, just call the vet to be safe and for possible treatment if he/she feels it's a problem.
Just want to reinforce it's probably not at all something to worry about, keep in mind what rookie said but I wouldn't lose sleep over it.