Hi Liza From an Australian perspective: In a formal situation we might say 'I'am suffering from a pain in my back' or 'I'm suffering from back pain'. More informally we might say 'I have/I've got/There's a pain in my back' or 'I have/I've got back pain'. More informally again and also idiomatic would be 'My back is killing me'. Hope this helps :)

Most people in southern California would say "I am suffering from pain in my back" or "Are you suffering from pain in your back?"