"Leave it" "big deal" etc.. do a 180....get her attention...distractions...put her focus on you....any apprehension on your behalf will be picked up by your doggy. Don't ever be the end of the leash to tighten up or pull first. I think at that age they might be getting to feel their "strut" for the first time....ensure your pooch there is no need to stiffen up...she's in good hands. And the age old..."it's okay" is not the way to go.
Yes, I agree with not tighten or pulling -we made this mistake.
You are entering the adolescent stage, and really need to step up training for about the next 6 months.
Think about rewards your dog likes, treats or toy. When you see a dog or person approaching, put yourself between your dog and the thing, get her attention with the treat or toy have a happy voice, when passing the person or dog, make a wide arc around them and give the treat, then when you have passed, give a happy praise. This worked for us, it was a bit tedious at first, but I would find busy places to take our dog for lots of practice.
Our dog is almost 4 now and stopped that behavior at about 18 months. On our outings will may encounter a creepy person, or crazy kid but my dog tends to walk away from them rather than go on the aggressive. Our trainer taught us that, sometimes people freeze. She generally is very good with other dogs, but its is constant practice, praise and rewards.