Forehead dent

#advicepls There is a dent on my 5 weeks old baby forehead. The dent shows up when he is crying. (When he is not crying, it is nice and smooth). He did not bump himself nor is he dehydrated as he eats and pee well. Any idea why this dent showed up suddenly? How long will it take to recover? What treatment or attention is required? Has anyone else have experience with this?