What Causes Pain In The Upper Back On The Right Side?

Ok. I am a very active and physically fit 63 year old muscular male (5 feet 7 inches and about 175 lbs) who has been active (working out with weights for decades, completed a few marathons, ran many shorter races and still run 30 to 45 mins a few times per week. My intensity in the gym is high and with heavy weights... But recently I think I have may have pushed it to hard on the seated row machine or some other back excercise and have developed a numbing pain in my righ back and upper neck area. Always have had crunching noises (decades) when I turn my neck and it always feels better when I stretch my neck but this is different. Usually my aches and paisn heal themselves in days to weeks but this one continues to ache. Also right sied shoulder/back area seems stronger than other side and I can see an imbalance (right side/shoulder is bigger and higher than left). What do yo think might be cause?