What Can Cause Black Spots To Appear Out Of Nowhere.

What can cause black spots to appear out of nowhere. 1 showed up since last night and they are almost like black scabs. And there is a almost like knot above 1 of them that is real tender and sore to touch. The males on my dad's side has a history of diabetes, liver and kidney problems and my father and uncle died from the flesh eating disease I can't
pronounce the name