Classical guitarist with neck pain on one side (feeling much larger muscles on the other side)

Going to see a physiotherapist next month. I had this paid few years ago and avoiding forward head posture and rounded shoulders plus some exercises sorted it, i eve got through a music degree without this pain returning. Now I have a desk job and I assume this is why I am not recovering despite being strict with posture and doing all the exercises I've been given before. This time however I also discovered that the muscles in the upper neck (just below the base of the skull) and shoulder on the opposite side to the pain are much more developed. Any suggestions would help be amazing!

PS: I play right handed classical guitar and the pain is on the fretboard hand; I've not been playing for few weeks now and it's not going away (although the pain was not present when I went on holiday mountain biking through the Lake District). I assume the pain is predominantly caused by 8 hours of desk time every day.