Before running out and spending a ton of money on a new bed, I would ensure that there is no other cause for her symptoms and if need be, have a Doctor look at her first. I would try to figure out exactly where her back is hurting and see if it is swollen, tender to the touch or warm. These symptoms could be an indication of swelling and/or an actual problem. I would also check for bruising and depending on the area, make sure that she is not peeing blood. Sometimes these kids, can get hurt and we aren't around when it happens and so we have no idea.
While Mommies intuition is almost always right, I would definitely ensure that there is no other cause before spending money.
Hope this helps.
N.