Probably some type of little bug they shared. I have that happen on occasion...either vomiting or diarrhea that lasts a day and passes from one to another.
It always makes me wonder where in the world it comes from. Just out of nowhere it seems.
Although....one summer I had a terrible time with my pups. I went to a photo shoot and they picked up something. Started out with Cookie and by the time the first two weeks were over, three of my pups had to be hospitalized for iv fluids. It was dreadful...some kind of nasty virus. That is why I am not keen on dog parks and meet ups. I am wary of strange dogs around mine.
I am sure it will be short lived like with Teddy.