Has there been a change? Did you look at pictures to see if it is new? Did you see a doctor?
You are posting where there are no doctors - we are patients like you.
I have uploaded an image of the skull depicting where exactly the lump is. Please go to the below link
http://img121.imageshack.us/img121/3035/skullbonestopview.jpg
In the image i have marked it in red.
That is the area around where i have the lump. Its hard and feels like a bone.
It could possible be posterior fontanelle.
I am not sure as i am not a medical expert.
Not everyone has the same shape of head - when you are born, things get squished up a bit and then flatten out - some people flatten out more than others. The skull is actually made up of lots of bones and early in life, it is not totally fused but as we get older, it fuses more - they tell the age of the skull by looking at that.
I would just ask your doctor next time you are there. If it is a new shape though, ask a doctor sooner. A way to know if it is new is to ask people in your family or look at pictures and see if you notice if it is new, or if you just noticed it.