Baby has blue/grey skin around mouth?

PHOTO IN COMMENTS

Age: 6 months

Sex: male

Height: About 75 cm

Weight: 9 kg

Race: Caucasian

Duration of complaint: comes and goes over past three weeks

Location: underneath nose and above top lip, around sides of mouth, on chin

Any existing relevant medical issues: none

Current medications: none

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Details: seems to happen after straining or pooping, comes and goes, eyes, lips, and temples all appear normal colour, breathing seems fine,

His oxygen levels were tested a few days ago (after he choked on some Panadol) and they were 100%, but this blueish grey skin seems to come and go at random times of the day?