I think I'm seeing things?

Okay, this is going to sound a little strange. For the past week, I've been seeing something out of the corner of my eye, it's roughly the same size as a house cat. I normally see it when I'm sat on the sofa watchin TV, next to my TV is a door leading from the front room into the hallway. Every now and then I keep seeing something walk up and down the hallway like I said it's about the same size as a housecat. I saw it again last night, but this time it was next to the sofa. Is my brain making me see things? Or do I have an overactive imagination? I had to sleep with the light on last night as I felt something was watching me, and I haven't done that for years.