jealousy fighting

Question
Jill,
my husband and I have a 2 year old Lab/Pointer mix, rescued about 6 months ago. She is a very happy and social girl, and gets along with most dogs. She has play sessions every day, sometimes twice a day, so she is very socialized and exercised frequently. She will sometimes just not like a dog, usually if they are being aggressive or overly sexual, and then she will not be shy about showing her annoyance and will give a quick snap, which usually ends things. But these incidences have been few and far between. I have not disciplined her for these incidents, as she was very skittish and shy when we first got her, no confidence, and I want to encourage her to come out of her shell.
So, last night, on her evening walk, we bumped into her favorite friend, and they started playing. There were a couple other dogs playing around too, and it seemed to me that the other dogs were very amped up, playing a little more aggresively than is warranted. Another dog came up to play, one that she does know, although not very well. This dog wanted to play with her friend, and then all hell broke loose. She attacked the other dog, not biting, but had the other dog down on the ground. The dog started yelping from fear, which I think startled her, but then she seemed absolutely crazed by all this and chased the other dog clear out the park.
She is fixed, her friend is a male, not sure if he is fixed and the other dog she attacked is an unfixed female ( a very submissive and fearful dog too).
I cannot believe the behaviour of my sweet natured girl. I don't know where this is coming from, jealousy? She's never, ever shown this behaviour before. She's very repsectful of my husband and I and knows whose boss, just am very worried that this is a trend. And if so, of what? And if she does this again to another dog, how do I correct it? I want her to get out and about and play and just be a dog, I don't want to be one of those owners that has to cross to the other side of the street because of my dog. Am I overreacting?
Help!

Answer
NEVER, for ANY REASON, "discipline" a dog for being a dog.  Your girl has (from your descriptions of former behavior) demonstrated normal dog to dog communication.  Play, in the dog mind, is not quite what we humans perceive it to be; it is a social interaction intended to establish and maintain rank.  Often, high ranking dogs will tolerate play behavior in a lower ranking dog that they would not tolerate in other instances.  However, two females (spayed or not) are almost always exercising rank perception during play (even when they live together in the same household.)  You do not really know what the "other dog's" intention was (although you report it as wanting to play), not having described the new dog's body signals.  It's quite possible the unspayed newcomer approached the male (your dog's "friend") in a manner that was not communicating an invitation to play.  You don't know this.  Your dog responded to something you did not perceive and behaved appropriately: she pinned the "offender" and then got rid of her (ran her off).  This is not aggression; this is not aberrant behavior; this is the communal interaction of dogs in a highly excited environment, responding to one another's body signals (which most humans can't read and which are quite subtle at times).  Her behavior toward other dogs (especially in a chaotic situation such as you describe) does NOT MEAN A THING about her interaction with humans.  Jealousy plays no part in any social interaction with dogs; it's all about communication (signals, intention, perceived rank, etc.)

You must be more cautious about these play groups in which you allow your dog to interact.  Multiple dogs in a free for all is NOT a social interaction that is acceptable.  If you perceive all out chaos in any such social gathering, or perceive any dog not behaving appropriately (remember that not every owner has the brains, nor does s/he care, if the dog is capable of normal social interaction with other dogs), simply step into the mix, calmly put your dog on leash, and calmly walk away.  Learn about observing dogs together and reading their body signals.  Short circuiting a normal social response in your dog will confuse her and possibly create an aggressive response toward other dogs.  Read Turid Rugaas' book on calming signals, Patricia McConnell Ph.D. and Roger Abrantes.