Please tell me what precise medical tests you have performed until now.
If you requested respiratory functional tests ... what were the results?

Hi NAC --- I also experienced this sensation. I think that, for me, the symptom was caused by anxiety. The best thing I did for this was try yoga, especially the practices for breathing which are called pranayama. The longer and the slower I learn to breathe, the more oxygen I take in. At least it feels this way. You can try this by laying on your back with you hands on your stomach. As you breathe in notice how high your hands float up. The goal is to learn diaphragmatic breathing, which will make your breath deeper. So with every breath in, try to lift your hands up a little higher and then totally relax on your breath out. Concentrate on breathing DOWN to your stomach and imagine the breath going to the bottom of the spine. Try 10 breaths and let me know how you feel.