Upper back pain?

I'm 18 weeks and lately my main syptom and quite possibly the worst is my mid to upper back pain. My lower back is fine and I hadn't done anything lately to hurt my upper back. I haven't even gotten that big, so I can't imagine what it is that's bothering me so much. I've only found one temporary solution and its sitting in a massage chair with the setting on "kneading". Has anyone else had this problem and found something to releave it?