Fearfulness turning into aggression

I adopted a dog from a shelter a little over a year ago when the puppy was 2 months old. She is a terrier Shepard mix with a little bit of some other things in her as well. She’s about 50-55 pounds the last time I checked.

She has always been really good with adults and children– sometimes a little too hyper... but always nice and friendly.

Around thanksgiving time last year, I took her to my parents house which I do every single week and she saw my dad wearing a bright neon orange hat and was immediately afraid of him. Even though the two of us were in my car and he was in the yard, she still was trying to crouch down and hide. My family thought this was strange because my boyfriend wears hats 24/7 and thought that once he took the hat off that she would be fine. It’s been a few months since then and even when he’s not wearing the hat she is still terrified of him and even tried to bite him.

Another issue I’m having is new and that would be that all of a sudden over the past month and a half, she has been terrified of new strangers. Normally she will pull me on her leash to go up and meet strangers because she is so friendly but lately she’s been jumping and is really scared of them. I’m a nanny for two children and she absolutely loves them but when she got introduced to two new kids who were my boss’s girlfriends kids, she was scared and would jump away from them if they tried to put their hand out for her to smell them and she wouldn’t take a treat from them. She basically stayed by my side and wouldn’t take her eyes off the two kids and if either of them made a sudden movement even if they were across the room on the couch, she would jump and crouch down to hide.

The newest incident happened yesterday with my neighbor. She has met my neighbor before and pulled me on her leash to meet her and was really good by staying low to the ground and rolling on her back and and she was really happy and friendly... but yesterday she was hesitant and would only take one step towards her and when my neighbor tried to pet her which she has done before, my dog was scared and quickly walked backwards.

My neighbor had two kids over who my dog had never seen before and they wanted to pet her and usually I would absolutely let them, but her fear turned into anger and she started having the hair on her back stick up. She was really good with one of the kids and let the kid put her hand up to her nose to sniff it, but the other kid she was terrified of and went and hid in the garage from her. The girl ended up quickly running towards my dog and cornering her into a wall which just made my dog even more scared and she nipped at the little girl.

This behavior isn’t like her at all. She used to be extremely friendly and would always want to meet people.. the biggest issue I had was her being too friendly and not knowing how big and strong she is. I don’t know if she had a bad experience with a certain person while I was not there and they were mean to her or if something else caused it or what. I took a trip to California at the end of June and I kept her at a boarding shelter by my moms house and so I don’t know if something happened there and that’s why she’s so afraid of people now or what. But that’s around the time when her behavior started changing.

Any help would be greatly appreciated!!