What Causes Itchy Spots All Over The Body?

Firstly it was only few dot at my back shoulders and it slowly spreads over my body skin. There's a change in skin like some kind of tiny round shaped dot appears and its different in colours. Its itchy sometimes and can find too much of dirt even as i took bath daily. Now its all over my chest, stomach, back shoulder, arms, thighs, necks and may be more parts. What's happening to my skin?