If the chest x-ray truly is normal, then this pain that only occurs with sneezing is probably due to a strained or torn muscle or ligament within the chest wall, around the ribs. Another possibility could be a pinched nerve in the chest, along one of your ribs. This type usually occurs after a fit of coughing or a blow to the chest. Finally, this pain could be due to a nerve being pinched, where it comes out of the spinal canal. X-rays of the spine can usually give a clue to this cause.
For the moment, you should give this pain time to spontaneously go away, and in the meantime, try to hold your chest with both hands over the site of the pain.
Have you been checked for a pulmonary embolism? My symptoms were pain in the left side of my chest (it sounds though like my pain was much worse than yours). It hurt me to inhale, cough, sneeze, laugh, etc. I was just diagnosed with a pulmonary embolism last week. They can do a blood test for clotting and a CT scan to confirm diagnosis.