Why does my girl need to sleep how she does?

Jul 2021
I recently got a girlfriend, and she has stayed a few nights over. When we are about to go to bed she wraps her arms around me, and as we fall asleep she digs her nails into my side or back. It isn't very painful, and I don't neccedarily mind, I just don't know why she does it. I'vw asked her about it, and she said she dosen't want to talk about it. All she would say is that it makes her feel better. I'm wondering if it could be the by product of some event in her past, or if it could be some kind of stress or pain release. Idon't know much about stuff like this, so any advice or thoughts would be much appreciated.