18mo son has diarrhea today, he vomited about 1 hour ago, and now a diarrhea diaper with a (single) long black fibrous bit in it. ?

Diarrhea: There is a possibility of a viral infection - stomach flu. This is also know as gastroenteritis. This is usually self limited and will resolve in 2-3 days with fluids such as pedialyte. The dark fibrous bit could be something that he ate. I would monitor his bowel movements and see if the dark fibrous bit recurs. If so, I would consider evaluation by the pediatrician.