Why does this happen? They usually go away in about 3 minutes.
This is known as the "sex flush," and it's very common, in both sexes. When you are aroused, certain capillaries (tiny blood vessels) in your skin can become dilated (they widen), causing more blood to flow close to the skin. Often the cheeks and forehead get a "flush," and this can happen on the chest and back as well, sometimes in irregular patterns as you describe. It's entirely normal.