“Why do I feel heavy lungs after having COVID?”

Based on your symptoms, you have probably had COVID PNEUMONIA. "Pneumonia" means that the lungs(mainly the alveolar air sacs--that are normally thin walled and empty) beccome filled with inflammatory cells and fluid. Thus the normally very light, empty, and compliant air scas become heavy and stiff--thereby resulting in symptoms that might include shortness of breath, difficulty taking a breath, a sensation of "heaviness" in the chest, cough, etc, etc. These symptoms commonly last for weeks to months and could be permanent(if the lung inflammation turns into fibrosis and scarring.)