What Causes A Blister On The Clitoris?

My 10 year old daughter noticed a small amount of blood in her panties while going to the bathroom. She called me in the bathroom thinking she may have started her cycle. There was no blood when she whiped, or anything later when we checked. She then asked me to look at her "peaches", because they felt funny. When I looked, there seems to be a blood blister like "something" on the tip of her clitoris. What is it? She says it doesnt hurt unless she crosses her legs real tight.