What Causes Black Spots On Abdomen, Back, Neck And Face?

I developed some black spots on my abdomen which spread round the back and upwards to the neck and face in the last 5 years. It starts with an pink or red inflammation, scales and swells into a black spot after 3 days. The spots almost appear sysmmetrically across the body devided into 2. Example, for a swelling on my left elso, there is also a swelling on my right elbo, on the left part of the chest, there is also another one on the right side. I have visted dermatologists and some said it is acne other eczema. What is the cause of this? Bhakti.