Hopeful,

Try to imagine this as similar to 60 Gs. Drop a 16 pound bowling ball 12 inches.

You would need to be moving very quickly to experience anything close to 60 Gs when you bumped your head under the car.

I've worked under cars for 50+ years and bumped my head plenty of times. I never experienced anything close to a concussion except when a 200 pound transmission fell 6 inches onto my head and smashed my head into the concrete.