I'm really concerned because Ive been checking my spo2 with a pulse oximeter every day, and its always around 98 or 99. But last night, I suddenly woke up and when I checked, my spo2 had dropped to 81. It went back to normal in just a couple of minutes. This is worrying me a lot because I had a chest x-ray done two months ago and everything looked fine. What could have caused this sudden drop?