I she just growling, or actually biting & snapping?  Was she isolated from other dogs for most of her earlier months?  If so, she may be feeling very uncertain & uncomfortable to be around other dogs.  Some dogs feel threatened by other dogs when first meeting them, & deal with it by going into defense mode straight off to hold the other dog away.  Some dogs just ooze dominance and she may be picking up on that & getting scared.  Or it may just be really overwhelming to be brought to a place where a bunch of dogs with high energy & lots of body language going on.

Some parks have a more rough & tumble group of dogs visiting, while others are very controlled with lots of nice mannered dogs.  I only frequent the first type since my dogs are not the best at dog parks either, and generally everything goes OK.  If there seem to be issues I just pack my two up & leave.  I notice a big diff in their behavior if they socialize on a regular basis, vs. when they haven't been around other dogs for awhile. 

Maybe your dog will settle down the more she gets out & meets other dogs & becomes more comfortable with pack dynamics.  Dog parks are kind of scary places though.  You have to be on your toes to judge if anyone has a really dominant, aggressive dog out there.  It may help to just keep her on leash for awhile & visit as many parks as you can to get her used to it.  But for some dogs, being on leash around loose dogs can also make them nervous.  Good luck with her, I hope you can get it figured out so all of you can have fun!