I always tell my patients to listen to their bodies, and in certain back conditions, sitting up straight may be even more painful than bending because of added pressure on the lumbar spinal nerve roots. Conversely, a patient may get some relief lying on one side in the fetal position.
There hasn't been much scientific evidence directly linking posture to developing back pain, but it makes sense that http://www.handingouthope.com/a-beginners-guide-to-mattress-buying/ posture can put pressure on different parts of your back. And if you're always slumped to one side, some muscles may become weaker than others and therefore more susceptible to strain or damage. If spinal supporting muscles weaken, it can also affect the position of your spine, contributing to back pain.