What Could Be Causing A Vertical Indentation Above My Left Eyebrow, Accompanied By Occasional Dull Headaches?

Hi, a couple of months ago I noticed a vertical indentation a little above my left eyebrow to my hairline and it’s only really visible when you stand next to a lamp or certain lighting. I feel it’s got worse visibly when I rub my fingers over it. The right side of my head is completely fine like how my left used to be and now this has come up. It’s accompanied by small dull headaches now and again but it can come in waves and the small headaches are only where the indentation is and nowhere else. I don’t remember banging my head or having a fall it’s just came up out of nowhere. I was recently in hospital for something else so had blood tests done anyway and they all came back fine, I’ve also been to the opticians and my eyes are healthy and the nerve that signals from the brain to the eye is very healthy and fine. I got a nurse to check over while in hospital and she seems to think it could be a slow growing cyst or growth on the bone of the side on the forehead but she said it didn’t feel like anything sinister but I’m still not sure about it. Have you got any advice as to what it might be as it’s stressing me out a lot.