2 days ago I noticed a marble sized lump on the back of my head. It's hard like a bone and attached to my skull. it's on top of that 'dip' where your skull goes to your neck - right in the middle. It's completely painless but I was worrying incase it was a tumour etc. I looked up some stuff on the internet and alot of people have this but with headaches, which I don't have. I recently had a throat/ear infection around a week before this happened and have been told by the doctor that I have Eustachian Tube Dysfunction which was caused by my throat infection. (all this happened before the lump). The lump is attached to the skull, I cant move it around but can move skin around it. And as I said, it's 100% painless to touch.

Any idea what it could be?