I’ve just joined the forum, so it’s the first time I’ve seen your post. I can see it’s had a lot of views, but am surprised that you’ve had no replies. I’m not a medical professional – but I would have been if I hadn’t developed M.E. when I was 21, so I have a great interest in illnesses.
My first thought, when you describe severe pain upon inhaling is Pleurisy. The reason this comes to mind is that my other half has it at the moment. Pleurisy is inflammation of the pleura – the pleura essentially being the lining of the lungs. When it becomes inflamed, it makes breathing painful, and can hurt across your back. It can vary in severity. My partner is being treated with Ibuprofen, to reduce the inflammation, and therefore reduce the pain, until the virus that’s causing it passes.
My second thought though, is that if you woke up with the pain rather than it developing gradually, that you may have slept funny, and damaged one of the muscles that supports your back. Depending upon where you pull the muscle, it can hurt when you breathe. You can either wait for your body to readjust, or have someone who knows what they’re doing, massage it. Personally I use an osteopath for such problems.
There are no doubt other things that can cause this pain. If you’ve not already done so, and you still have the pain, see a doctor.