Wheaten suddenly aggressive towards other dogs?

My Wheaten always loved other dogs, she's always been super sweet to everyone. Recently things have changed...if she cannot say Hi to another dog, she goes crazy, will growl and bark but her tail is still wagging. (I kind of think shes pissed she cannot say hi, but Im not sure)

Last night she went to meet another dog and her hair stood up and she rounded her back and tried to go after him (well they tried to go after each other)

Ive never seen anything like it. Im not sure if I should keep her away from other dogs now, or if this was a one time thing?

The ONLY thing thats different is we are starting to walk her with a front loop harness and are keeping her on a short leash because shes a big time puller.

she's 5, if that helps.