Then it hit me, the pain in my back wasn't due to the baby, but to vibrations from the machine going right up my back. Our rental house was so cheaply built that a toddler walking across the floor sounds like a 400 pound man is jumping up and down from underneath. The light bulbs in the basement have to be replaced every month because of the effect of this vibration. Since then I have put a carpet pad under my chair and a pillow on my chair with no effect.
I don't want to stop quilting, especially right after buying such an expensive machine and frame, but I don't want to risk permanently damaging my back if I continue. Luckily this isn't going to be our residence forever and I will ask the builder up in Maine to reinforce the joists or whatever needs to be done to make sure I can quilt in the future.