We just went through this - it's not fun. With Tessa, they never did really figure out what's causing it although my vet said that with the sudden hot weather they've seen an awful lot of this.
If she's going frequently with diarrhea, a small dose of Pet Pectin should help. They sell it at PetSmart and it's like kaopectate for dogs. 1/4 pepcid twice a day may also help. No food for a day to let her little insides rest, then rice or rice with a little bit of boiled hamburger or chicken. My vet also said a very small amount of plain noodles might be a nice change for a bland diet. You'll also want to keep her from being dehydrated, so a bit of chicken broth might tempt her to drink.
If the diarrhea keeps up, I'd go to the emergency vet as you don't want her to get dehydrated.