What Causes Appearance Of Reddish Spots On Forehead?

three years back i had chickenpox. I noticed 3 to 4 round flat reddish-brown spots on my body.(they may have been there before the pox too that I did not notice). After sometime they became brownish black. After that whenever i have fever these circles become reddish and restore their black color after fever subsides. Off late such patches have started to appear on my forehead. what could they be?