You have to have a chest xray done, bottom line. It could be nothing other than a pulled muscle and good chances thats all it will be, but you have got to find out.
You said in the thread title that you had shoulder pain too?-Pain in the right side also with deferred shoulder pain is a common symptom of gallbladder issues. Gallbladder issues are common in Cf and very easily treated.
As others said, pain that bad is present with a lung collapse, pneumonia, flu, and also with pulmonary embolism (a clot).
Not trying to worry you!! Just to not ignore it.
I had a bout of similar type pain several months ago, while I was pregnant on top of it. I had recently had a bad virus so figured my lungs were still trying to get over whatever it was.
It got worse and worse, I tried everything to relieve it. My worrying got worse too, finally I went to the hospital and the docs thought I had a clot. After many tests-it turned out to be a gallstone and constipation that was giving me all that pain! (and I'm pretty tough...this pain was awful) But they said basically never ignore pain that bad.
I'm sure its nothing major-go and get some help with it. <img src="i/expressions/face-icon-small-smile.gif" border="0">