sounds like it might be swimmer's ear. Water gets into the ear canal and causes an infection. You can tell if it is swimmer's ear by exquisite tenderness to the tragus- the little thick flap-like protrusion that you can press on to close the ear canal. Even pulling on the ear lobe of someone with swimmer's ear causes pain.
If it's an inner ear infection, it will hurt further in and touching the ear, earlobe or tragus won't change the pain. She could also have what's called an external ear infection... (which actually may be the same thing as swimmer's ear-- I'm not sure on that)
If it's swimmer's ear, they give ear drops that really do help it... and you keep the ear drops in case it happens again.
Some of my friends put rubbing alcohol drops in their children's ears after swimming to help evaporate any water still in there, to help prevent swimmer's ear.
If it's an inner ear infection, they need antibiotics- no question about that.
Since this is the first instance, you'll probably have to go to the doctor to get a diagnosis for swimmer's ear. Afterwards, if it happens again, you can always say- "she has had this before, it hurts when you touch her ear, I think it's swimmer's ear, we're out of drops, can we get a prescription..." etc.