I started developing these marks on my left arm. they have only shown up on the one arm. the colour was a light brown, it has faded a little over the last few weeks but has turned almost a black colour. it is coming from underneath my skin.

Doctor can evaluate: Rashes are very difficult to describe in words, so an exam by a primary care doctor and/or a dermatologist is helpful, when the rash is present. Sometimes rashes occur from bug bites, fungus or yeast, reactions with chemicals, irritants, plants, allergens, etc... If one cannot go to the doctor, then it is helpful to take a set of photos to show a doctor later, or to show an online doctor.