Kay, is there a possibility that she had sat or slept or her tail. And teh weight() or nails somehow caused it to do that? I'm not sure if that would be a reason...but it could.

Or she could have bumped, but never noticed the pain(since she was busy having fun and not focusing on her tail)until she was lying down.

Poor girl...hope she feels better soon, I'm glad that it heals within 2 or 3 days!