2003 GMC Suburban battery not charging sign

while i was driving my GMC 2003 suberban the battery sign appeared and a sign saying battery not charging and the battery gauge went down to 12, then i continued driving and it went back to 14 , after a while it went back to 12 and the sign appeared again, this happened four times and i continued driving, thought it might not appear the next day, but still it did apear again, what do u think the problems is ? please