Thanks for your question on Health Care Magic.
I can understand your concern.
Since you are smoker, we need too first rule out Pulmonary causes like
bronchitis,
lung infection etc because smokers are at increased risk of lung diseases.
So get done chest x ray and PFT (Pulmonary Function Test).
Chest x ray is needed to rule out lung infection.
PFT is needed for the diagnosis of bronchitis.
Smoking cessation should be done as soon as possible in any case.
You may need antibiotics, inhaled bronchodilators and inhaled corticosteroid (ICS).
If all these are normal than rare possibility if
Pulmonary embolism. You are taking hormonal contraceptive(nexaplanon), so risk of Pulmonary embolism is high.
So
consult pulmonologist and discuss all these.
Hope I have solved your query.
Wish you good health. Thanks.