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Often the darkening of abdominal skin is due to a problem with the thyroid hormone being too low. Along with that can come a yeast infection of the skin leading to the odor you are smelling. You can read about thyroid issues in dogs here
I'd suggest having your vet check her over to see if the problem is her thyroid. A blood test will show if that is it and there are meds that can help with that.
Another possibility is an adrenal gland problem causing Cushings disease but a thyroid issue is fairly common and your vet will likely check for that first.
Hope this helps you!