I noticed a large bruise under my Apple Watch after a 5k

I have a similar problem - I am 71 and often get these bleeds after a knock but never after simply using the watch. I can get them simply getting something out of my pocket. It's an age thing, dare I say.

I would swap wrists while the bleed takes a few days to dissipate, make sure the watch is snug but not too tight, and if the other wrist does the same seek medical advice about continuing to use such devices.

See advice in changing Watch orientation here.

Change language and orientation on Apple Watch – Apple Support (UK)

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