I find it a little amusing that you have just asked on a forum whether you are just feeling the back of your scalp lol. If you have never noticed it being this large before, then I would seek advice. I wouldn't worry though.
I have actually had times where I have been feeling around my head and started worrying about lumps and bumps. I also feel across my forehead and it feels like there are little cracks in there. If you poke and prod, feel and touch your body for too long, you will inevitably start to find things lol. Maybe just stop playing with it and ask other people who can actually see you? Ask someone you know if they can see what you think you feel.
But other than that, I wouldn't worry about it too much.