Welcome to HCM!sHORTNESS OF BREATH
on climbing stairs and during high physical activity has number of medical causes especially when it is accompanied by chest pain
,confusion or dizziness
.most common ones are heart diseases,asthma
,COPD and Anemias.
As in your case you have got a thorough medical examination done to rule out these medical causes and your shortness of breath is not accompanied by any other symptom so in my opinion this may be due to less of routine physical activity . you can start a regular exercise regime and see if it works!