A fecal is a test the vet does to look for parasites and other baddies at your request. You just need a fresh poop in a baggie and bring it to them for testing (call first, though, LOL!).
Electrolytes will help your horse with all the diarrhea. Nothing fancy is necessary. You can even make them at home much more cheap:
26 ounces of NON-iodized salt
22 ounces of Lite Salt (potassium)
2 Tablespoons of Epsom Salt
Give no more than 1 tablespoon a day. If you soak food, it is easy to mix the powder in the water, then add the feed, and then feed it (once a day) until her diarrhea is resolved. Otherwise you can just moisten the food and sprinkle it on, but I would do 1/2 a dose 2x a day because she might think the food is too salty - or she might like it.