You have every right ot be portective of the dog you are walking. Pitts are capable of killing, so even though lots of breeds can be aggressive, Pitts can be especially dangerous. Especially to a small dog.

If you're really feel that bad about it and feel a need to justify yourself, you could speak to the neighbour without dogs and let her know it isnt personal, you just have had bad experiences with other dogs harming dogs and you want to be careful because you just never know.

But honestly, you don't need to justify it, i'm sure the neighbour knows the reason. I used to have a Doberman and people would cross the street to avoid us. I didn't take it personally, I found it kind of funny because that dog was a big baby.