old, or lack of exercise?

i'm not sure if i'm getting older or if it's due to a lack of exercise... but during the past week and a half, i've been having rotator cuff issues. it feels as if i have deeply bruised my collarbone right at the rotator cuff or pulled/ overexerted the muscles and ligaments around the shoulder. and just this morning as i was out in the field taking some saddle measurements, i strained something just below my shoulder blade.