My 3 Yr Old Hit Her Head 2 Weeks About.

My 3 yr old hit her head 2 weeks about. It awoke up and she had a knot on her head for about 2 days and bruising for about a week. When the bruising subsided I noticed she has a hard slightly raised line on her forehead where she hit it. When she furrows her brow it looks like a wrinkle. At first I panicked and thought it was an indentation but it seems to be a build up of something under the skin because I can feel it move when I touch it. What is it and is it dangerous?