4 month old puppy barks at other dogs

So I have a four month border collie X yorkie pup. About two weeks ago she started barking at other dogs, previously she was absolutely fine, ignored dogs. It has escalated from little boofs to quite loud barks- which only happened today. I can usually redirect with treats but if the dog gets too close or barks she starts barking. I can still redirect when she is barking but her attention only lasts a few seconds. When the dog is calm she redirects fast and stays focused on me but when the dogs are moving fast or barking at her she struggles to stay focused. Is this normal? Its really stressing me out and I'm scared I've made her reactive. Should I continue to redirect with food and toys? We also have another dog who she is completely fine with, and she is fine with dogs we know. Today for example she did great with most of the dogs we saw until a group of four dogs passed us very close, and she barked a lot, then after that an offleash dog ran up to her and she barked and even a few little growls. Also another puppy was barking at her and she barked back but in that instance I think she wanted to play. Please help!