How can I stop my back from hurting when I meditate?

I sit zazen in burmese posture on a zafu, put my hands in mudra on my lap, and keep my eyes open and unfocused, towards the floor. I try and do it first thing in the morning, when I have not had a chance to build up tension, but no matter when I do it my lower-to-mid back begins to hurt about ten minutes in. I am not keeping my back unnecessarily straight, nor slouching. Could the problem be the height of the zafu or where I’m sitting on it? Help!