Hello My Husband Just Noticed A Raised Spot On The

Hello My husband just noticed a raised spot on the top of his head closest to the back. I looked at it and it appears to be raised approximatly 1/8 th of an inch, is circular and is almost 3" across. He said he didn't bang his head or anything and is tender to the touch. It is slightly red, Any idea what this could be?
Thank you. L. Gold