Were you cutting anywhere near there? Is it possible she just got a little cut from squirming?
One of my pigs got repeated nosebleeds, but it wasn't a drop and then stopped... when she had nosebleeds, they BLED. We did see the vet but never got a really clear diagnosis, but they thought potentially polyps or even a malignancy in her sinus cavity (though nothing showed up on imaging.) She was lived a good six or seven months from the time of the first nosebleed so I doubt it was a malignancy, and she died of unrelated causes at around 6.5 years (so an elderly guinea pig.) I would see the vet if it recurs, but hopefully this was just a minor thing that won't recur!