Hello ! Yes pneumothorax is definitely one of the possibilities . However it is not the only possiblity. Since you mentioned shortness of breath and chest tightness, even bronchial asthma is a possibility. Have your ever had similar episodes in the past ? Problems related to your heart cannot be ruled out just coz you din t collapse. The oxygen levels drop in the higher altitude so your shortness of breath could have been a response to that as well.You need to be examined by a physician in person . You will need a Chest X ray , ECG , Spirometry (pre and post bronchodilator) for basic cardiorespiratory evaluation. Treatment needs to be decided based on your final diagnosis . Do not miss to consult coz if it s a pneumothorax(air getting collected around your lungs) it needs to be drained. So do book an appointment with your physician at the earliest.Do quit smoking too ! Your health needs priority.