Gliding Black Blur

This has happened twice, but I will tell you guys the two separate incidents.

The first time happened 5 days ago. I was sitting right here, at my computer in my room.

I will give you a layout of my room. When your standing in the doorway, right in front of you is my bed, to the left is my bookcase, computer desk and TV. On the wall that is right behind the bed, are my two windows.

On Thursday night, around 7 or 8 pm I was sitting here, talking to my friends on MSN and listening to music. Usually at this time my mum comes into my room and fixes up my bed and closes my window. Obviously for her to do that, she has to walk behind me. As I was typing to a friend, I saw a black blur walk/glide really fast behind me. I thought it was my mum, so I paused my music and turned around to talk to her. When I turned around I saw that no one was there. I yelled out to mum, and asked her where she was. She said she was in the kitchen. There is no way that she could have walked behind me, and then somehow got to the kitchen, that fast. I told her what I saw, but she just shrugged it off.

Then, 3 days ago, on Saturday night, this happened again, but this time, the black blur walked behind me again, although this time, it was at around 9-10 pm - mum was asleep. Also, it moved a bit faster.

My mum and I are very close, so the next morning I sat mum down and told her about what I saw again, this time she believed me.

She then told me hen she was my age, she saw spirits and ghosts too. She and I also share a psychic ability, I can sense when something bad is going to happen before it does.

What do you think the black blur is?