Hello Thanks for trusting Healthcaremagic. I have gone through your query and I can understand your concerns. During physical exertion is there will be dilation of your blood vessels and this is the prime reason for flushing and redness. It is a normal response and nothing to worry in it. In your case only one side become red. It may be because more neurotransmitters are more located on that side. You can consult a dermatologist if it persist for long. Wishing you good health Thanks