Well it's not the lungs, that's for sure. It could be some muscular pain or anxiety. Is this pain perceived on the left side or right side? Back or front? Does it hurt when breathing? All the time or just sometimes? You have free healthcare in Germany, you should be able to see a doc. Or just go to emergency and say you feel bad, they will do some checkups for free. Though it usually takes a few hours until someone will see you, if you're not an emergency.