Back pain from office chair

Oky so I'm currently 25w going on 26w pregnant. I work in an office all day sitting on a basic office chair with only a partial back part to support my upper back. (Of course a full back chair would be more comfortable but I don't have one). I just want to ask other women who have worked in offices all day long through pregnancy for some advice to relieve some of my back pain? Ironically the pain (feels like stiff muscles)is in between my shoulders and ribs, my lower back is not sore at all. I've tried stretching and getting up to walk regularly but the pain persists.