On/off burning feeling in mid upper back. seem worse when lying down.started a desk bound work for about a month now.wondering if it is cause by that?

I would put money: on it. Your body is most comfortable when you are moving about and your muscles/joints/spine are in various and changing positions. When one is static, muscle strain results; it can feel like burning. Try to get up and stretch/move around every 45-60 minutes. Begin a stretching program, after you have warmed up, a few times a week. Warm baths. Massage. Try a few things to develop a helpful program