First, any sudden change in behavior means a vet check is in order. Pain can make dogs grumpy and mean, and illness (like thyroid issues) can change behavior. It's worth a basic vet check to rule out physical problems in my opinion.
Then, I would start with keeping them separate unless supervised for the time being. Most likely, something DID happen earlier that day or week and either you (or your mom) didn't see it happen or didn't understand that it was something that was a problem.
When you say "attacked"- was there blood? Did she do damage to the other dog? Were you able to separate them verbally or did it take physical force? Where were the dogs right before the attack? Were they both sleeping? Were there any toys or treats around? How old is your mom's dog and are they both spayed? Has she been snarky or snarly with your mom's dog before and just never physically attacked?
When you say that she doesn't get along with dogs that she doesn't know, do you mean she is aggressive towards them? Reactive only while on-leash? Or just prefers to avoid them or only reacts to other dogs if they do something like bark at her or rush up to her?
Then, I would start with keeping them separate unless supervised for the time being. Most likely, something DID happen earlier that day or week and either you (or your mom) didn't see it happen or didn't understand that it was something that was a problem.
When you say "attacked"- was there blood? Did she do damage to the other dog? Were you able to separate them verbally or did it take physical force? Where were the dogs right before the attack? Were they both sleeping? Were there any toys or treats around? How old is your mom's dog and are they both spayed? Has she been snarky or snarly with your mom's dog before and just never physically attacked?
When you say that she doesn't get along with dogs that she doesn't know, do you mean she is aggressive towards them? Reactive only while on-leash? Or just prefers to avoid them or only reacts to other dogs if they do something like bark at her or rush up to her?
