What Causes Nosebleeds?

Hello. I have a three year daughter who has started having nosebleeds quite often. She is almost 4 years old and began having the nosebleeds when she turned 3. I would estimate she's had about 30 nosebleeds since she turned three. The nosebleeds are always only in her right nostril, never the left nostril. They usually occur while she is sleeping, but also occur while we are at the grocery store, driving in the car, or while she is at preschool. They are most times heavy, but stops within 5-10 minutes. We run her humidifer every day and night and we have begun putting vaseline in her nostril. We have spoken to her pediatrician several times regarding her nosebleeds, but he indicates no real issues and has suggested saline drops to put inside her nose. Today, the school nurse called me and said she has had three nosebleeds today! That has never happened before and I am becoming alarmed. I am not certain what else we can do to prevent the nosebleeds. Should we be concerned?