So here is some information on your questions. Hopefully some of it will answer your questions.
Candidiasis of the Vagina (Vaginal Candidiasis, Vaginitis, or Yeast Infection)
Symptoms may include itching, burning, and pain around the vagina, labia (vaginal lips), or anal area and thick, whitish, sometimes curd-like vaginal discharge. Usually, you know when you have it. So it may be coming from another source not her vagina. But infections are caused by:
Antibiotics, steroids, birth control pills, and foods high in sugars or starches (breads, pastas, and alcohol) all promote the growth of Candida
Douching reduces levels of "good" bacteria in the vagina and is not recommended
Alcohol and nicotine (present in tobacco products) promote the growth of yeast
Because yeast grows best in moist areas, wearing looser-fitting pants or underwear can help prevent yeast infections. Cotton underwear breathes better than underwear made of polyester or nylon, and can also help reduce yeast growth.
Here's some info on dogs; yeast can be normal in dogs mouths so it could be contaminating the underwear. We can also have yeast in our mouths in low numbers too (normally).
Yeast infections are not sexually transmitted but can be associated with oral sex. So you could actually transmit them to her that way and then infect yourself.
So this is a complicated question.
Also, you could be getting it from just sweating during sex. The moisture may be causing you to get yeast or another fungus like joch itch. So, I wonder have either of you actually been diagnosed with yeast? It could also be a dermatophyte (which causes joch itch and athletes foot).
Since the dog is chewing her underwear it could come from that and it would a good idea to keep her unused underwear in the hamper and the dog out of the bedroom perhaps. As the article says give the dog a bone.
So maybe it gets on her skin and not into the vagina and then gets on you. But hard to say. Unless you handle her underwear and then touch yourself. These things can happen without thinking of the route of transmission.
If you have any other questions feel free.
The article was removed but basically the information I gave you is the same thing. By the way a dermatophyte is a type of fungus and as I mentioned causes skin, nail and hair infections like joch itch and athletes foot. One other thought is that if you are using an over the counter medication or even a prescription one yeast are becoming resistant to the ones that end in azole. Such as fluconazole. Usually you can see the chemical name in the ingredients. So you can use terbinafine (Lamisil) if changing out some of the habits above don't work.