That is a little bit odd. Perhaps she is infatuated with you? Maybe she is just enamored with the view from your window? A lock would not provide that knowledge or safety. I would put a small string between the door and the frame, on the hinges side, and close the door. If the door is opened the fallen string will let you know that she is going in your room. Then I would obtain a small video camera. There are dozens of sites, geared towards spy gear, that sell inexpensive video cameras with several that can stream to your phone. Some of these cameras are activated with motion sensors. This would let you see what it is that she is doing if/when she invades your privacy.From there you need to determine if you are in danger and/or this room mate is a good fit for you.