My dachshund tried to eat a small frog a few days ago and had foam in her mouth, now she has watery eyes and some stuff in corner of one eye. So maybe your dog came in contact with a frog.
Um, take it to the Vet would be a good idea!!!
Take it to the vet as soon as possible because it may have a serious disease like rabies and if you don't take it to the vet soon then it will die