It is very common!
Whether it be true inflammation or extra mucous that doesnt allow the 02 saturation.
Usually when the 02 sat is low for anything length of time, it takes a bit of time & intervention to get it back.
The exception that I know of is when Zoe4Life had her surgery. The anesthesia made her sats drop, but after being on some 02 supplement for awhile it bounced right back.
For me it also depends on the position my body is in. My 02 sat is always better laying down. The doctor say this is odd since your lungs cant expands as well laying down versus sitting up. I explain it that for ME I am more relaxed laying down. More relaxed equals relaxed airways etc.
They thought I was nuts, but I have proved it to them over & over again!