Why is my shoulder burning while I'm on the computer?

[WaveyK]

I am constantly sitting at my desk playing games or talking to people on my computer or instagram, sometimes 8-15 hours a day depending if I'm not busy. but every time I sit my left shoulder starts to have this burning feeling. I've tried cooling the burning down with cold lotion or applying an ice pack, but it keeps going. I'm only 17 and this has happened since I was 16 or late 15. It's really strange, though. It only happens on my left shoulder and left side of my back. Once I get up, the feeling is still there but gradually goes away, and will completely be gone in 2-3 hours. Can someone please explain this?