My daughter hit her head and now there is a "sunken" spot on her head.?

My 4-year-old daughter fell out of her chair and hit her head on the floor (wood floors). She doesn't have any symptoms of a concussion, but the spot that hit the floor is red/purple and when I press on it, it sinks in. The Dr.'s office is already closed so I can't ask them until tomorrow. Does anyone know if this is something serious? I don't think I've ever seen a "bump" that is going in and not swelling outward. Is this something to take her to the ER over? I just don't want it to be serious and I'm sitting here doing nothing. She is only complaining of her head hurting. She also seems tired, but that could be because she hasn't napped today.