My Daughter Fell Off A Bench In A Restaurant And

My daughter fell off a bench in a restaurant and landed forehead first on the tile floor. She cried immediately but I noticed a round indentation at site of injury. I took her to the ER but the Dr said since she was acting fine there was no need for concern and refused to do any imaging. My question is why was there an indentation and how long after a trauma could it take for symptoms to present themselves. And my daughter is 17 months old.