Make sure she's in a cool place and bring her body temperature down by wetting her coat and paws. If she drank a lot of water in one go, this could make her sick. Let her stomach rest for half an hour, and then offer her small amounts of water and wait to see if this stays down. You can also dab her gums and tongue with a damp towel to make her more comfortable. If she continues to vomit, then it's essential to see the vet.