Sometimes, when there is excess mucus secretion in the airways of the lungs, you get an "air bubble" feeling on deep breathing.
You may be feeling easily tired because the mucus in the lungs may be preventing enough oxygen
from being exchanged in the lungs and therefore inadequate oxygen circulation in the body.
Visit your family physician or a chest physician to diagnose the problem. 3 weeks is quite a long time so it's better to get yourself checked rather than keep feeling worried.
All the best!