Dog park behavior?

My dog was a rescue, but I've had him since he was 3 months old. He was extremely shy and skittish in the beginning and even tried to bite me the first day. Overtime he got better and better and it seemed like we were making progress every week.

We went to the dog park for the first time at 5ish months and he hated it at first, hid between my legs, growled at every dog. We went rarely, but occasionally so he could get used to being around other dogs.

Long story short he was doing amazing, absolutely loved the dog park, always happy as could be, didn't want to leave. Playing well with smaller and larger dogs, this happened for months on end.

One day at the dog park, he started barking and growling non stop, all i could see in the direction he was barking was a lady trying to drag her small dog out of the park by the collar. Even after this lady left he would not stop so we left. He was in my sight the entire time, i know he wasn't attacked or anything.

I've tried returning to the dog park 3 or 4 times since then with no luck (over the course of about a month). Its worse than when we first started going. We didn't even make it in the gate 2 of those times. His hair stands up, and he growls, barks, and just acts like an attack dog anytime another dog comes within 6-7 feet.

I've tried bringing another dog around him outside of the dark park and he does the same thing, but after I tell him no a couple times its like it kicks in thag the dog is not a threat and he's tail wagging wanting to play.

Does anyone know what may have triggered him or how I can go about trying to resolve this? Ideally I think I'd just do the above scenario as often as possible with single dogs but I have no relatives or friends with dogs. Thank you.