Tell the invigilators (with some sort of proof) and I'm sure they'd let you have a pillow, or help you change to a more comfortable desk (I did similar but had broken leg, got in 10 mins early and got to hand pick the best desk :P ) Also make sure you sit up straight, get good sleep night before and do everything you can to improve concentration (water, not hungry but not full up ect) and then just be ready in the exam to spend a total of 5 mins readjusting and worrying about back pains. If you tell them, you may get 5 mins extra. I got 5 mins extra in all my GCSE's (didn't need it, but it helped me relax so I guess I needed it emotionally xD ) because of my cast ect.