Hit my head on the street yesterday. i have a mild concussion. headache and dizziness better today. have a stiff neck though. should i go to er?

Yes: You must rule out the possibility of bleeding into the brain. There is a well known lucid interval that may last for hours following a head injury. It would be wise to go to the emergency room and rule out the possibility of a sub dural bleed.

Get checked: It is best to get checked as soon as possible. Don't think that you can 'sleep off' a concussion. It's better to be safe.