Hey there!

Well, questions (Note: best answered to yourself as brain storming): Did you have a small accident lately? Like falling, bumping into something solid? Were you engaged in sports of any sort, prior to last nights incident? Any "wrong" moves you made?

I only heard of this kind of pain in combination with breathing in terms with a (slightly) broken rip or injuries to the rip cage area, hence my questions. It also could be a sprained muscle, but this is most likely not the case.

I suggest you go and see a doctor asap, just to be sure and since you speak of pain every time you breath, I do not encourage you to exercise/work out/participate in sports.