what is musculoskeletal pain and what causes it?

[StellaCorona]

So DP suddenly developed a bad stabbing pain in his upper right side last year. An ultrasound found gallstones but at his follow up appointment the consultant said the pain was not related to gallstones and was in "musculoskeletal" possibly exacerbated by poor posture and that DP should take paracetamol.
Why would this pain suddenly appear? DP has not changed his posture radically.
Is it possibly something related to arthritis and should he have been tested for this? He is 66
Anybody have similar and can suggest something which actually helped?

It's an all encompassing phrase that means it's bones or muscles but nothing sinister. I have arthritis and it doesn't sound like that, much more likely to be posture or an injury.
If it's continuing then I would ask to see a physiotherapist.
DS2 had similar, he is very tall and had poor posture plus he was hunched over a desk studying for exams. Physio was very beneficial.

Thank you MyFriendFlicker. That's helpful.