short of breath

hello i am 38years old, generally healthy only basic flu's, headaches. i used to take methemphedimine ( i have been clean for 1 year now) i get short of breath i used to only get this when i had been using but now iv been clean for the last year i have recently over the last 6months started experiencing this again,
i have to yawn to get a full breath. my chest feels tight and restricted.
i have just had a ecg test at doctors today, came back normal, he listened to my lungs they seemed ok. and iv just been for bloods (waiting for results)
i am stumped to what causes this i do not suffer from anxiety, iv never had panic attacks.
i can be short of breath just sitting doing nothing or if i am exercising. i do not wheez or cough so asthma haas been ruled out.

i hope someone can help me discover what is casuing this