Agreed,, pop her back to the vets, perhaps a 2nd opinion? What are you feeding her and what have you tried?

Although she may seem ok in herself, having diarrhoea will make her dehydrated so with this heat ensure she has plenty of cold water (my boy has a water fountain at the moment to encourage drinking in this weather) and a cool fan to pass out infront of if she wishes.

Ollie also likes ice cubes (filtered water) in a bowl so he can lick them .... and then knock them out and play with them and cause a lot of mess ..... but thats another story