Can computer posture (leaning in, slouched) cause chest and upper back sensations/pain/discomfort? if so, how to fix?

Maybe--fix posture?: This may not pertain to you, but it's amazing how often patients say the equivalent of "doc, it only hurts when I do this." slouching and poor posture can certainly make you ache. If you can fix your posture and feel better, there's your answer. It's a good idea to find someone who can help you with good "ergonomics": proper position of you, keyboard, monitor, etc. Look it up online, too.

Posture is important: Sitting at a desk (most office workers) is both unnatural and detrimental to your skeleton. From the neck down to your lower back, the spine is confined to an awkward position that can cause ligament and muscle strain, c costochondritis (inflammation at cartilage bone interface), and likely pain while this is possible, chest pain should be checked out by your physician to rule out other problems.