I feel for you. I have a mix breed who resembles a pitbull - if she actually has any in her is up for debate because she resembles at least 5 breeds in various ways. My other dog is a shepherd/dobie mix so he has that black mark also simply due to his looks.
When I'm out camping, I know it's just a matter of time before there's an incident and my dog gets the blame regardless. If I see a dog off-leash in our vicinity, I stop, sit my dogs, and ask the owners to get their dogs. The owners always get b**chy with me about it, but I just stand there and say "take hold of your dog NOW". When they finally grab their dog and I walk by, they make comments about my dogs - I simply respond with "and there was no problem here was there - once you controlled your dog as required".
If the other dog is on a leash, I simply step off the trail and put my dogs in a sit while they walk past. That's usually enough for the other owner to get that I'm not interested in doggy interaction.
One suggestion I would make is to not use the "small dog" areas regardless of her actual size. Always use the "large dog" areas. Owners of small dogs are usually more nervous about dogs that may resemble pitbulls and that can lead to complaints like this. Many don't see their dog's growling as a threat because they're so small - but see your dog's growling as a threat because of her looks. Owners of larger breeds tend to acknowledge that both dogs had a bad reaction and just move away (they may justify to themselves later that YOUR dog was obviously the issue, but they usually don't make official complaints about your dog).