@jrofri So many possible causes, from serious to not serious. Personally, left shoulder pain was something I experienced in hospital after undergoing HIPEC with cytoreductive surgery. In my case it was trapped wind in my stomach.

I hope you get to the bottom of yours and get peace of mind, as well as relief
https://www.bimuno.com/news/gut-health/6-ways-to-ease-painful-trapped-wind/