I think excessive thirst is a sign of liver malfunction - something to do with blood filtering... (I think)
You should call your vet. A medication given to your dog could be causing a bad reaction.
Maybe a side effect of her medicine just make her feel thirsty.
You can ask vets at this site for free:
Umm my dog who has just turned 11 had a similar condition. Does your dog have bad teeth, or had teeth recently done etc? Because my dog does. Because it hurts, he pants, as it means the sides of his mouth won't be touching them. That way, it doesn't hurt. Because he pants he loses lots of water, making him drink more, thus, weeing more. Hope it helps !!
There are a number of conditions that cause increased thirst in elderly dogs. I wonder if your girl is neutered??? If not one of the signs of womb infection (pyometra) can be increased thirst and this is very common in older bitches who have not been spayed- especially if they have never had puppies. Other conditions which should be checked for include kidney disease so really you should go back to your vet and ask him to take another look at her- especially if she seems unwell.
Hope that helps.