Please find a vet who will analyse your dog's urine to see if she has a urinary tract infection. If she has a bad urinary tract infection she will need antibiotics. There are plenty of reliable supplements which can provide adjunct therapy but nothing works as quickly as antibiotics, and since urinary tract infections can be painful I would treat her as soon as you can. Cranberry juice or capsules are a good supplement for the urinary tract as a preventative and adjunct therapy.
The only reason I can think of for your vet to dismiss this problem is if your vet believed that your dog was going into her first estrus (heat) and believed that the discharge (pus) was a normal estrus discharge (or period), which of course is possible at her present age. But if your vet had believed this he or she should have explained it to you so that you could have understood.
Lastly, if you are not going to breed you dog please have her spayed. This can really eliminate her chance of getting breast cancer.
Ummm, since when is puss in the urine something not to treat (or at least investigate further)?? Puss is a sign of infection and infections, left untreated, can be serious.
My dachshund has small crystals in her urine and has had urinary tract infections (hey, she's 10, she's due).
Per my vet's recommendation, I give her two cranberry capsules each day (one morning, one night) in "pill pockets". I also give her two tablets of Omega3 & Omega6 fish oil at the same time - no pill pockets requires, she thinks they are treats. I get both products from the "human" health food store but you can also find them in the vitamins and supplement area in most pharmacies.
Just my experience.