A typical cause of hairloss on the back end is a flea allergy. Usually cats with a flea allergy will have scabs
on the skin, too, so this may be less likely in your cat's case. But since she goes outside, I would definitely treat her with a good flea preventative, such as Frontline or Advantage, to make sure a flea allergy isn't the problem. (One flea bite is enough to cause problems in an allergic cat, so you don't always see a live flea or flea dirt on the cat).
I would also be concerned about an endocrine disease causing the hairloss. The main ones in cats are diabetes
and hyperthyroidism, although Cushing's disease is also possible. All 3 of these diseases can cause excessive drinking and urinating. Generally cats with these 3 diseases will also have very good appetites and, in the case of diabetes and hyperthyroidism, lose weight. I would expect the underlying skin to be normal, so these fit a bit better with your cat's problem. Bloodwork would be the first step in the diagnosis of these diseases; Cushing's disease can be suggestive in regular bloodwork but would require further testing to diagnose. All 3 diseases can be treated (although Cushing's disease is more difficult to treat in cats).
Other allergies can cause hairloss with normal skin underneath. Pollens, molds, house dust, etc can be culprits, as could an ingredient in the diet. If your regular vet thinks it could be an allergy, a steroid injection would be given. Because a steroid injection can increase the chance of developing diabetes, I would do bloodwork first!
Finally, some cats will pull out their fur due to a behavior problem--boredom, stress, etc. This hairloss tends to be symmetrical and the underlying skin is normal. This is hard to diagnose and other problems are ruled out first.
So I recommend taking your cat to your regular vet very soon for some bloodwork and an exam. If there is an underlying problem, the sooner you get it diagnosed and treated, the better off your cat will be!
Hope that helps-