12 Year old boys

For the most part I have a wonderful son, National Jr. Honor Society, Archery Club, Guitar, Tennis, all A's & B's and respectful to others and pretty cultured.  On top of all that he does his chores which are quite a bit for his age. (Take out the trash, recycling, does his laundry and puts it away, makes his bed, cleans his room and the bonus room, dusts, puts his dishes in the dishwasher, etc.) But then there is this side of him that has changed from my sweet little boy to this 12 year old unrecognizable kid.  Who is he and where did he come from?  The one who talks back or gets mad at me when I ground him.  I have to remind him to do things a thousand times.  When did this happen? Why me? Most of the time he is the child who loves to cuddle on the couch with me and watch a movie or play a game or go do some fun activity, but then out of the blue comes this thing that does not like what I tell him to do and poof there goes my child.  How do I get him back, where does he go? Does anyone have anwers to my questions? HELP ME!!!!!!!  I miss the little boy in him. I do not want to spoil him and cater to every whim and want him to learn to be independent and thankful and not be spoiled like some kids I see. I don't want to be like the parents who over compensate and never make the child have consequences and they act spoiled and the parents life has become all about the child. Those are the parents I hear others talk about and look at them and pity them how sad it is.  It is a struggle, but seeing some examples of these spoiled children I swear I will never be like that and my son has to learn consequences to his actions even though it is hard as it is to do as a single parent.  I feel awful, like the worst mother on earth sometimes, but he needs to know it is not always what he wants and not all about him. Do not worry this is a rare occurance when this happens with my son, but still I am not used to it and hope it is a disease that goes away someday....... how much longer will I wait for this to happen? Just venting.......  :)