I don't know. I suppose you'd want to watch it for any future changes.
Thank you for yor reply, NTB.
I went to see a nurse yesterday, and she checked my nail, blood pressure, temperature, and my pause, and all the things were normal. She suggested me to take a blood test and transfered me to a doctor. The doctor said that when people catch a cold, lip gets the stain like that.
I am still a bit worried, though.
I pressed and pinched the stain spot, but nothing had changed.
I wrote that the stain had been like that from before, but I do not think I had stain like that, though my lip was always towards pink rather than red.
I suppose the first thing to suspect would be not enough oxygen: cyanosis. If you pinch a fingernail area, does it blanch but then go right back to its normal color when you release? What if you press on the lip spot?
OTOH, if the area is always exactly the same, maybe it's some kind of birth mark, like the port wine stain type.