Sorry this is so long!
I am new to the forum here and looking for advice related to one of our dogs exhibiting what seems to be selective aggression when off leash. Here is the background: We adopted a rescue dog 2 years ago (neutered male, mixed breed, 25 lbs, approx 8 yrs old). Then 6 months ago we took in a second rescue dog after their caretaker (a family member of ours) passed away. She is a spayed female, mixed breed, 35 lbs, approx 6/7 yrs old. We had never met her and didn't know what to expect, but by all accounts we felt like we won the rescue lottery twice. Both dogs are sweet and mild mannered. They are house trained, well behaved indoors, don't jump, aren't runners, and rarely bark. Our male dog is nervous around strangers (retreat response), but our female dog is a cuddle monster and seems to love all people. We also take them on walks every day and both dogs seems completely fine and calm meeting other dogs on leashes.
Here comes our issue:
Our female dog (the one we have only had for 6 months) recently had 2 separate aggression incidents that have us very worried. The first incident was 1 month ago when we took her to a friends house to meet her dog (100 lb male). The dogs seemed fine at the fence but immediately started fighting when we allowed them in the yard together (no puncturing but it was very aggressive). We separated them and reintroduced on leash. Everything seemed well so we let them off leash again and the fighting started right away. We took her home after this since none of us felt equipped to resolve the issue between the dogs and we weren't sure what even happened to trigger it or if one or the other instigated it. We basically decided that we humans didn't handle it well.
The second incident took place yesterday. A friend came to stay with us for 2 nights and brought her dog (65 lb female). We had done some internet research this time so first we introduced ourselves to her dog. Then we introduced her to our dog. Then we brought them together on leashes and walked them around the block together. They were sniffing each other and enjoying their walk overall. It seemed really positive so we brought them in the yard together off leash. Everything seemed fine and we were ready to label the previous incident as a one off, but about 10-15 minutes into it, our dog laid under a chair and we noticed she seemed anxious. We were discussing bringing her inside because of this but then she came out and proactively attacked our friend's dog (no puncturing but it was aggressive). We picked her up and immediately both dogs seemed 100% fine. We separated her into our bedroom for the night and plan to keep them completely separate for the 2 nights they are staying with us.
However, we feel confused because (1) our dog has met other dogs in the 6 months we have had her, though we now realize that they were all smaller than her and these two dogs are bigger than her (as well as more hyper), and (2) we had even taken her to a dog park a couple times - only twice because our winter was really harsh here this year but one was a very popular restaurant/dog park that had 40+ dogs of all breeds and sizes present. There were no incidents at all, not even a hint of an incident at the dog park. She seemed happy and was exploring and we were very pleased, but as you can imagine we won't be taking her anywhere off leash now.
I researched trainers in our area and being in a small market, I only found 2. We plan to call them both this week but I am just desperate for additional resources or advice, especially in the off chance that we can't afford these trainers (one only does boarding/training and the prices seem very steep but we haven't talked to them yet). Has anyone had a similar incident and been able to resolve it long term, or do we have to keep her separate from other dogs from outside our household forever? Is it possible it is only bigger dogs? Any thoughts would be very appreciated.