I have that thing too but only when i get hurt so badly, like big painful wounds, and after the pain or the wound heal, it vanish, and i found that natural
I get the same thing just like you do random. You should go to a doctor it could be meny things. When I went It was just because it ran in the family. He said that it could have been meny things and it was good we came to him.
But mine just appears out of nowhere:( anyways thanks for the answer :)