Nose bleeds

I'm wondering if anyone else has encountered this:

My gf has started getting nose bleeds occasionally (happened twice so far) and we can't explain why. At first we thought it was because of a space heater in a small studio that she was sitting too close to but the second time was not in the same environment. I've read up on a few sites but nose bleeds aren't listed as a symptom. Anyone else experience this before? Is there anything to worry about if they hadn't happened before?