Brief Answer:
Can be due to some other cause

Detailed Answer:
Hello mam,

Welcome to healthcaremagic.

I understand your situation and concerns.Since you said your oxygen level is good(hope you have checked it with pulse oxymetry), there can be several other reasons which is causing this smothering.

Without knowing your present status properly like ;
Are you a hypertensive patient?
Do you have any pre existing heart disease?
Are you on any medications?
Are you passing good volume of urine?
Do you have any lung disease?
Are you having cough ?

it is hard to give any confirmed diagnosis.But looking at your age, i would first think of a congestion in heart which is causing difficult to breathe and making you tired.I would suggest you not to delay and immediately see your nearest physician and discuss about it.

Meanwhile;
Do not drink lots of water.
Try to stay in an airy place.
Do not do any kind of exercise.
Avoid stress and anxiety
Stay away from any allergic things around you.

If i would be your doctor,i would have done an electrocardiogram,chest xray and try to rule out certain diseases. And would gone for further investigations as required.

Hope i could clarify your doubt.Feel free to ask more.I would be happy to help you.
Dr.Moumita XXXXXXX

---