Hit my head very hard on a car trunk 5 hrs ago - no symptoms, but now have a sore spot on one side when I touch it & also hurts when I bend over. Ok?

Contusion. most likely. Put a cold pack on the sore spot so you don't get a lump, and don't take any aspirin (which can increase the bleeding). If you develop dizziness, nausea/vomiting, or difficulty concentrating be sure to see your doctor. Soreness should slowly resolve over the next few days.
Could just be bruise. but of course we worry about concussion, you should see a doctor.