Pain under right breast

Some clues that might help are what makes the pain worse/better, is it constant or does the intensity fluctuate, has it responded at all to any intervention (heat, cold, massage, medication)? The studies you've had will see most structural abnormalities (bones, muscles, organs) but not nerves. The distribution of your pain, WHERE it hurts, matches or makes sense for pain coming from one of your spinal nerves that run on the under-surface of your ribs. Those nerves can register pain in your brain if they are injured (blunt trauma to the area, turning too fast - not that you remember), pinched (herniated thoracic discs are pretty uncommon though), or infected (ever had chicken pox? was the pain preceded by a rash in the area?)
If you pulled a muscle, then using that muscle would make the pain worse, same for a bone injury. A partly collapsed lung, which can happen for no reason at all, will hurt more with breathing, might make you short of breath, and should have been easily seen on the CT or X-ray.
Hope this helps.