Golden Retriever sudden aggressive behavior

Question
Hi! We adopted a Golden Retriever/Cocker Spaniel mix (mostly retriever) from the animal shelter about 10 days ago. The dog is about 18 months old, and has been as sweet-natured of a dog as you'll ever find. Two days ago, however, the dog jumped and nipped at my 11-yr-old son's face when he laid down next to her. We thought the dog had been startled, and that was the reason. Yesterday, however, my son and a friend had been playing (with large sticks, which they use as 'swords') but had sat down. The dog - totally unprovoked - began seriously growling and then barked and lunged at them (I had her on a leash, so no harm done). My husband and I are very surprised -- we've only heard her bark twice in the time we've had her, and the only somewhat-aggressive behavior has been toward our two cats (which we've kept separated until all have time to  adjust).  We have reason to believe the dog may have been mistreated before being in the animal shelter.  She was picked up as a stray, but we're pretty sure the dog had some previous owner, as she is crate-trained, housebroken, and knows basic commands. A curious note - she doesn't know how to play.  She just watches a ball roll by her, and doesn't romp and play as most dogs will. She enjoys taking walks but spends a lot of time sleeping or lazing around.  Do you have any idea why she would suddenly turn so aggressive to my son (even after the sticks were removed from the room she was growling and trying to lunge)? My son and the dog have been very close until this episode, and understandable his feelings are very hurt right now.  Any thoughts or suggestions would be greatly appreciated!

Answer
A lot of people have problems like this will adopted dogs. so you aren't the only one! the only thing i can thing of is you need to get your son around our dog more. let them be around each other as much as possible, but have your son not pay attention to her until she gets completely used to having him around every where. she needs to know that he is the one on top. if she misbehaves have him put her in her crate or where ever she goes. and have him tell her no if she does something bad and soon she will get it that he is on top.       *hope i helped*

                                -Madison