My 6 Month Old Has A Soft "air Like" Bubble

My 6 month old has a soft "air like" bubble on the top if her head ( just past the forehead). I just noticed it the other day when looking at her head and thought I saw a small raise or bump. When it touched it, it was very soft and I could push down on it and see it "pop" back up! What is this and should I be concerned?!