That is really scary! If the hives don't diminish soon, you can try giving her an oatmeal bath, or if you have some aloe&oatmeal shampoo on hand, that could work too. These will help with itching. For now, just try to get her to calm down. Lay down with her and see if you can get her to lay still and keep her heart rate down. I'm not trying to scare you, but it's better to be cautious in a situation like this.