Hit my forehead above my left eye on the bathroom door pretty hard. It is swollen, red, black & blue. I didn't lose consciousness but saw stars and was/am dizzy. Head hurts too. Should I go to ER?

Or your doctor.

Hitting your head on the door will likely cause pain and dizziness. The fact that you didn't lose consciousness makes it less of a concern.

Sadly we really can't diagnose online, so it's up to you. If you feel you need to see a doctor, you should. The ER is for emergencies. Unless you're slipping in and out of consciousness because of this hit on the head, you shouldn't bother the ER that are dealing with real emergencies, and having to treat things like this makes it less likely for people that really do have an emergency, to get treatment.

If you're conscious and only have pain where you hit your head, and dizziness because of that, you can make an appointment with your regular doctor. But the decision is yous. You have to decide if this injury is in need of emergency hospital care