Hello, My 16 Year Old Gadget A Knot That Has

hello, my 16 year old gadget a knot that has one upon her head in the back right at her hairline it came up about 8 months ago, it was tiny then and she never said anything else about it, she came to me last night in pain with it it has gotten to be the size of a golf ball any idea what this could be? And it hurts to touch it no headache and it is soft to the touch it's not hard