I have back pain too, mostly in the upper back between the shoulder blades, but sometimes in the mid back, left side too. I'm on Nexium as well. I never had back pain before all this started, but I thought it was due to the reflux, not the medication. When I first started the Nexium, I had pain in my lower ribs, in the front, but that went away. I heard that was a possible side effect of the Nexium but I didn't connect the back pain to it.
Kassie