In patients with "breathlessness" or shortness of breath, there are three most common organs that should be checked and this needs laboratory examinations. First of all, are you obese? are you diabetic? Is your breathlessness triggered when you are doing strenous activities or it occurs even when you are at rest?
If you are obese and your breathlessness is triggered by activities, then the cause can be cardiac in origin. I know that you are asthmatic as you've mentioned that you had an asthma attack before, then I think you have an asthma in mild exacerbation now but I believed that there is still another contributing factor on this. Check your blood pressure and your serum cholesterol level. If you are hypertensive, then you have an increased pressure to your heart which is a cause of breathlessness too. By chest xray, you can also see if your heart size is normal or enlarged. If you have an enlarged heart accompanied by hypertension, then you have a hypertensive cardiovascular disease which needs medical attention. Another organ that can cause breathlessness is the lungs. The most common is asthma and COPD. The latter can be found in patients who are active or even passive smokers. Passive smokers are those who are not smokers but they are surrounded by people who smoked. The finding of this in chest xray is the same with asthma which is hyperinflation of the lungs. Another thing also is pneumonia, this can also cause breathlessness and this can be seen also in chest xray.
The kidney also will give us breathlessness or shortness of breath which others described it as shallow breathing. This is already a morbid condition and this patient has a failing kidney that needs dialysis which I don't think that you have this. This can be checked by doing a serum creatinine level.
Patients with very elevated blood sugar level will also have a shallow breathing or acidotic breathing . In this patients, you can smell that they have an acetone breath and this can be found in patients with Diabetes.
So now, you have an overview of the possible causes of breathlessness. For you, I suggest to have your blood pressure checked as well as do the serum cholesterol level determination, serum creatinine, ECG and chest- xray and let the results be seen by a doctor for proper assessment since diagnosing an illness needs physical examination too.