I am a 28 year old male, 170 lbs, 5 9 (in my second year of physical therapy school). I am a runner - usually 30 to 40 miles per week at a 6min to 5:30 pace. I have been running my whole life without symptoms of chest pain. About three weeks ago while running in a cold environment (about 20 degrees F) I experienced chest pain around the 4-5 intercostal space on my left side. I thought it was related to cold oxygen and stopped for a couple of minutes. The pain subsided and I continued running. I returned to the treadmill a couple of days ago and experienced the same pain at a 5:45 pace. In the past 4 months I have done VO2 Max testing and mile testing without pain. This is a new onset. I am a nonsmoker and drink rarely. I eat healthy. I know my next step is for diagnostic testing and to see a specialist I was just curious if you might have any idea what s causing the pain?