I'm so glad I found this subreddit. Reactive puppy problems.

I have an almost 5 month old german shepherd puppy who has been a dream up until about a week ago. He has played with other dogs, had so many interactions with friends and family, and even stayed with a friend for the first time for a few days with no problem. All of a sudden this past week, he is so reactive to people and dogs - pulling on his leash, lunging, and growling. I don't have any idea what changed, but it has been the absolute worst week working through this with him. We start puppy classes tonight and I'm honestly dreading even going because I know what a handful he will be. I've been looking into prong collar training and muzzle training, but there's so many opinions it's dizzying. I know I play a huge role in this and I'm trying so hard to stay consistent and positively reinforce good behaviors, but I really want to trust my dog again. Thanks for reading this rant 😂