I had that exact same pain on my left upper back around 30 weeks. It lasted about 1 week and then disappeared. I found it was aggravated if I slept or napped on a soft surface (i.e. our soft couch) and lay on the sore side in bed. It is horrible because it hurts so much, even when you walk. I would try a heat pack if I was you and try sleeping / sitting in a different position than you have been the last couple of days. I hope that yours, like mine, if short-lived. Take care x