My 3years old fell 5 months ago bumped the back of her head leaving her a big knot the knot still there and she bumped again yesterday she say hurt?

Not to worry: It is common for toddlers & young children to be a bit clumsy, and bump themselves often, even on the head. Bruises on the scalp often cause a large knot ("goose-egg") because of the rich blood supply there, and this will be tender and can take a few weeks to heal. As long as she doesn't seem dazed, extremely sleepy, vomiting or sudden behavior changes, repeated bumps on the head should heal fine.