What would cause a dent to form in the middle of my forehead? (photo)

A few months ago, I noticed a dent forming in the center of my forehead which kind of looks like I was hit with a ball ping hammer. I have never had any trauma to my head to cause this. I have noticed it changes shape and grows pretty quickly. Is this something to worry about? The first photo is before the dent and the 2nd is after I noticed it.

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