I hit my head while falling back during soccer. at the time my head hurt a bit but not much. two days later the right side of my neck is sore.

Monitor: this is a type of contra coup injury with whiplash injury of the neck; however, if you lost consciousness, have double or blurred vision, nausea, vomiting or fainting, you must be examined. I suggest examination and x-rays of your neck.

Neck strain : It is not uncommon after such an injury to strain your cervical and trapezial muscles. I recommend moist heat, gentle massage and naproxen. Please seek medical attention asap if any symptoms of concussion or worsening neck pain / reduced range of motion.