I would say call the vet out. The diarrhea is very likely from the fresh grass(mine had a bit at first too) but it's better safe than sorry. I wouldn't worm her until the vet checks her out, just in case.There are also shots that she will need since shes pregnant. Do you know if you have fescue grass in your pasture? When pregnant mares eat fescue it makes the placenta very tough, tough enough to need scissors to get thru it. Have you ever had a mare foal before? If not, I would recommend putting an experienced friend(or the vet) on notice, it happens very quickly and if something goes wrong YOU have to act quickly.