There's a burning sensation under my skin in the upper back and shoulders. it get worse when i work as i sit on a chair with no back support. could it be inflammation in the muscles ?

Yes : While inflammation is the most likely culprit, poor posture could cause muscle,strain in this area. You don’t mention how long the problem has been present, or what makes it better/worse. If you are otherwise healthy, without systemic disease, then try bringing your shoulder blades together, hold for 10 count, relax for 10,count, and repeat 10 times, several times daily. Topicals can help