Have you discussed this with your docs? You need to make sure that you don't have a chest infection of some kind, or a problem with your heart. I don't want to alarm you, but any unexplained chest pain should be checked out by a doc. By unexplained I mean anything that is not associated with stretched muscles because you lifted something too heavy. I am not a doc and I don't know your medical history, so please get your chest checked - make sure the doc is aware that you have just started on CPAP.
I don't have my pressure set that high. However, 2 things come to mind from what I have read. The pressure, and the fact that you are probably breathing more deeply because of it, could be stretching the chest muscles more than they are used to being stretched. That could cause the chest pain. Secondly, you may not be moving around as much in your sleep anymore, and that can put additional strains on your back. If the problems are caused by breathing/not moving, you will probably get used to them in a few weeks.
Diagnosed September 2009, Respironics System One w/ C-flex and humidifier
Aloha nasal pillows
Not a medical professional, just a patient with severe OSA (31) who has done a lot of reading.