I Have A Sensitivity About Eight Inches In Diameter On

I have a sensitivity about eight inches in diameter on my left side. It is only at skin level. No soreness from under skin level. My GP did x ray which was negative. Not shingles. He said to monitor and if it becomes more sensitive he may send me for an MRI. I am a 68 year old male in excellent health. Exercise daily. Not over weight. Don't smoke. This is not a muscular injury. I am not a hypochondriac but I have never had a sensitivity like this before. Any thoughts?