Unfortunately, the only way you will find out what the problem is, is by seeing a doctor.
As you are able to lay on your stomach and take a deep breath, then it is not your lung causing this problem.
It is more likely to be your liver, gall bladder or a muscular skeletal problem. Did you have a fall or did any physical exercises or pick up or move any heavy objects that may have triggered off this pain?
Take pain relief medicines as recommended on the packet. Massage may help if it is a muscle problem.
could also be your stomach causing the pain.
where exactley is the pain, i thought my pain was my right lung turned out to be my gallbladder and liver causing the pain. do you smoke? and does it hurt to breath in? you really need to get some blood tests done to rule out your pancreas, liver and gallbladder also your appendix can refer pain to under your right rib. then go from there. im not sure how medical works where you are but in nz you would never be turned away from the hospital for pain. i wouldnt leave it, probably nothing serious but ypu do need to rule out the obvious then look into getting an xray of your lungs because it could be anything. good luck. shar x