It's from moisture trapped against the skin because the band isn't breathable. Take a sniff of your band. It gets nasty! Our skin isn't meant to deal with plastic up against it. You may find a non silicone replacement band or a silicone one with a lot of holes eliminates the problem. My dedicated sport watches have a lot more holes in the band to let moisture out. Also wear it looser and keep the skin dry under the band. If I wear mine to bed, I put on a cloth band I bought from Amazon.
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