It could be worms. I have seen it just like you described. My friend who breeds malteses had a litter, one puppy did that poo one day ... 3 days later everyone including mommy had runny poop. Now i can not remember if it was Gyardia , coccidia, or else??? It has been at least 4 years. Anyways, i would treat it sooner rather than later. She had a little work nursing them all back to hard stools, besides puppies are frail anyways one should always be careful.
You could try doing the pyrantel, and doing the "panacur" for 5 days, it will kill most of worms except for the coccidia. For that one you will need a prescription.
The only problem with trying to treat just at home without being sure is that if it is not worms, you may be delaying treatment , which could turn out more costly and harder to fix.
Good luck
XOXO
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