Welcome to healthcare Magic!
The feeling of smothering at breath may be due to respiratory problem or a cardiac disease.
The respiratory causes commonly giving symptoms are Hyperreactive airways or exercise induced asthmatic attack when you have a sudden onset of bronchospasm
and difficulty to breath.
Cardiac problem that can cause trouble like yours are- congestive heart failure
, a very high blood pressure
surge leading to cardiac failure state, or a transient cardiac rhythm disorder called arrhythmia
You should get a medical evaluation for the appropriate diagnosis because the treatment depends on the exact diagnosis.
Your diagnosis will be helped through a chest X Ray
, ECG and ECHO to start with and further plan can be based on the report of this work up.
I hope the advise would be informative and useful for you.