Thinking caps. Pleurisy ? I was struck down with pleurisy a few years ago now. The indescribable pain. Breathlessness. Fatigue - and all the rest. Went on for months - took several months to get over. See a Pulmonologist.
Costochondritis. Can be extremely painful. Also breathlessness. (I suffer from this). See a Rheumatologist.
Vascular spasms. Pain. Breathlessness - will NOT show up ECG nor on X-Rays. Might well have inflammatory markers show up on a blood test. (I also suffer from this. Darn painful). See a Cardiologist.
​Courage to the sticking point - you are going to get on top of this. Go fight em - you'll win.
I've had similar problems in the past and a few different things helped me out.
First off, do you suffer from asthma, COPD, or any respiratory illness? Are there any areas of your health that are not considered to be up to par? (Weight? Blood pressure? etc.) Do you smoke or are you often exposed to unusual amounts of smoke, pollutants, toxins, chemicals etc.?