Why is it that I feel nauseated after hitting my head?

Eval. Symptoms of a concussion can include: headache, nausea, vomiting, confusion, blurred vision, slurred speech, weakness, feeling dazed, numbness, lack of coordination, problems with short term memory, dizziness as well as repeating things over and over. You can develop a concussion with or without loss of consciousness. It may be prudent to be evaluated.
Brain senses it. Nausea, like all other sensations, is really the brains interpretation of inputs from throughout the body. The nausea centers in the brain are commonly disturbed in a head injury. Watch out for progressive/ worsening nausea - could be a sign of something causing increased pressure inside the cranium, and needs medical eval.