Hit my head pretty hard on the doorway. when should i start to worry?

Right then: Cheer up. If you hit hard enough to get an epidural hematoma you'd be dead already. The other clot, a subdural hematoma, develops more slowly. If you have a bad headache that gets worse, anything that seems abnormal, nausea, vomiting, etc. Go the er where you will have a ct scan that will probably be normal, an enormous bill, and be told you have a concussion. Worry if things are getting worse.