[Elizabeth]

Alright. I just wanted to know if she was younger, because then my advice would be different.
She most likely knows what was going on. However, you shouldn't be embarassed that you were masterbating because it is natural. Next time, lock the door and try to do it in a more private place.
If you want to talk to her and feel comfortable enough, then go ahead. If she brings it up, then you need to let her know you will try to be more private about it.