foal with diarrhea

Hello everybody, I'll introduce myself properly in a minute!
I took in a rescue shetland pony, 8 years old and apparently healthy. What I didn't know is that she was also pregnant! Her foal, a big filly, was very bright and healthy, easy delivery, good nurser, no apparent problems. She's eleven days old. Today she had sudden onset diarrhea, pretty bad. Very liquid, covers her whole butt right down to her hocks. Still running around and nursing normally.
I forgot to mention, mom had all her vaccinations for a pregnant mare, but not wormed since her fecals were totally clean.
I read about foal-heat diarrhea, should I assume that's what this is, or should I have the vet see her right away? I read that if it isn't foal-heat diarrhea, it could be something rapidly fatal. I am a novice horse owner, don't know much. They are pastured, by the way, on about two acres of grass (and thistles).
Thanks for any help you can give, I'd be totally devastated if anything happened to my baby!

The timing could be foal heat diarrhea. Check the mare to see if she is in heat.

If EVER in doubt call the vet.

Definitely could be foal heat scours. But do give peptobismal to help settle the tummy. Also check to see if your mare is actually in heat.

Another common cause is Ecoli bacteria. This is resolved by giving antiobiotics for 7 days.

Thank you both. How do I tell if the mare is in heat? Discharge?

We give the foals plain yogurt.

To tell if a mare is in heat, basically look behind her and see if she "winks"at you.

If I were in your situation and unsure about what it is, I would definitely call a vet! Better call a vet and it's "nothing", than don't call it and it's fatal!