I sometimes have difficulty breathing, like my lungs feel "heavy". No cough or colds & I havnt had an asthma attack for years now. What could this be?

Difficulty in breati. You have a past history of bronchial asthma and it can be an attack of asthma with or without infectious process like bronchitis or pneumonia.The fact is asthma can always come back if there is a trigger.You need to see your doctor and need a pulmonary function test after clinical evaluation, and xray chest exam.Then you can be treated with inhaled steroids, bronchdilaters with/without antibiotics.
Many possible causes. There are many possible causes for the symptoms you are experiencing. Asthma is certainly high on the list, especially since you've had asthma before. However, a sensation of "heavy lungs" or chest tightness can be caused by multiple lung conditions or even a heart issue. You should see your doctor as soon as possible.
Further eval. You need further evaluation with a history, physical examination, breathing test and x-rays. There is a number of different possibilities, asthma is just one of them.