Seemingly feeling better

I took Watson out for a walk before crating him to go to work this morning. He came out of his crate and sat for the leash. We walked down the block and he seemed a bit more energetic. From what I saw, I think his lethargy might have had to do with stomach upset. His stool was very liquid. Anyway, we went on and walked down the block (which we have walked a couple of times) and we were passing this fence where 2 dogs are out in the backyard. These dogs always jump on the wood fence and bark when I come by with a dog. Watson hadn't made anything of it the last time or 2 we passed, but this time he turned toward the fence, walked part way to it and stood there. The hair on his entire spine went straight up (piloerection), his legs were stiff and he starting barking at them. This was something new and not expected or desired. I call him away and we walked on by and his hair calmed down. The concerning thing, other than the aggression to the dogs on the other side of the fence is that he seemed calm as we walked another block, but when we came upon a little girl coming out a backyard fence, the hair on his spine came up a bit. She went back in the yard and we walked a few paces past there and turned and I told him to be nice and she came back out and he was wagging his tail. This aggressive side warrants watching.