Dog suddenly aggressive at dog park

My 1-1/2 year Beagle mix is very socialized. Been going to dog park for over a year, perfect with 3 kids, likes strangers and other dogs.

Suddenly is acting more tentative and nervous and when we went to the dog park this weekend - twice - first acted snappy when a dog came up to the kids upon entry but then calmed down and played happily for over an hour with all dogs.

Today, I had the kids come in after her so that the dogs didn't approach them immediately, but she went right for another Beagle, so aggressive I had to completely restrain her and remove her before she bit. While restraining her, in "attack" mode, she DID NOT act aggressive toward me in any manner.

Is she going into an overprotective mode over her "Pack"? I'm now very nervous to ever try the dog park again. Also worry this action will grow while in the neighborhood.

Thoughts?