What Causes Spots On The Skin?

There is a spot which looks like a rash/pimple but it s very round and doesn t continue and connect to others it s on its own and it has a dark red/brown/black spot right in the centre. Some are itchy, but most are not. I had this before and it was all over my back and legs and stomach. But it was one different spots on its own. I know for a fact it isn t the chicken pox because I have had that before, or hives (looks nothing like it). It s not like a regular rash either or pimples at that fact. What is this? What can I do to prevent it from happening again? What may be causing it? Thank you!