Why Does My Dog's Butt Smell Like Fish?

He might need his anal glands expressed.

You should see your vet. Sounds like her anal glands may be over due for an expression and may be becoming infected or impacted and need treatment. You should see the vet this time since there may be an issue beyond just needing to be expressed but in the future might consider a regular trip to a groomer who often includes this with their services (a little more bang for your buck) and having it monitored by your veterinarian if that's her problem. Hope she's feeling better soon.
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Sometimes it smells like that when they need their anal glands expressed.

Your dog might have anal gland problem that can be impacted, infected or there can be tumor. You should take your dog to vet for proper diagnosis and treatment.