Ghost Pretending To Be Girlfriend

I was staying over at my girlfriend's house and woke up at about 4 a.m. I looked up and saw a face staring at me. It looked like my girlfriend with her glasses on, but the face was a bit different and kind of creepy. I just stared at it for a few seconds but it disappeared. It seemed like she was standing through my girlfriend, my girlfriend was laying in bed and whatever was staring at me was standing through her and the bed. I checked to make sure my girlfriend was still sleeping next to me and she was dead asleep.

I kept it to myself for a few days and then finally asked her if she was staring at me sleeping that night, and she said that she wasn't. Also earlier that same night me and her performed a small seance, but nothing really happened during it. What is this pretending to be my girlfriend? And why would it do this?

And also, about a week before this I had an experience at her sister's house. Me and my girlfriend were staying over at her house and we were sleeping on the couch. The couch is facing her sister's bedroom door. I woke up in the middle of the night and saw a girl standing in front of her sister's bedroom door. It looked like a teenage girl, but I couldn't see her face. Her face was either dark or she was facing the door, I couldn't tell though. Could this have anything to do with my other experience? Or could something be following me or my girlfriend?

I know my girlfriend and her sister have always been sensitive to the paranormal and weird things happen to them quite often. But my girlfriend said she has never experienced something paranormal trying to look like her so were both confused about the situation, so please give us some advice.