I have tight upper back muscle. haven't stretched in a long time.always sitting down. i have indigestion quite a lot.are these due to inactivity?

Hard to know: It's possible you're having some type of referred pain from your stomach or esophagus, going to your back; and at least 2 separate things could also be happening simultaneously. Do you have a chronic illness that interferes with being able to move? You're only 18? It sounds like you need a medical assessment to be more sure of what's going on with your complaints.