A yeast infection will cause the skin to get darker in some places. Vets call this condition hyper pigmentation. Changes in color are also caused by friction. When pets lick of scratch the same area over a long period of time, because of allergies for example, pigment producing cells in the area get overactive, and the skin looks darker.
A number of the body's hormones, including those produced by the thyroid and pituitary glands, play a large role in regulating melanin, the skin's natural pigment. Pets with hormonal imbalance may produce too much melanin, causing the skin to get darker in some places.
But ,,,Go to your vet and get it checked out!