Were you treating the abscess on your own or under the care of a veterinarian? My vote is that the abscess was far worse than what was visible on the surface, and likely needed surgical drainage. Antibiotics can do only so much, and if they weren't specific for the particular bacteria strain at work, they would do no good at all. I'm guessing that the infection got into her blood stream (sepsis) and she finally died of multiple organ failure.
Most dogs don't make a whole lot of noise when they're in pain. We can't judge their pain levels as we do with humans. Unless a dog is very vocal, they're just extremely stoic about pain. Case in point, my old dog who died of pancreatitis, which is an extremely painful condition. Part of the exam involves the vet squeezing the dog's abdomen to check for a pain reaction. All he did was arch his back a bit and take a deep breath. The vet said most dogs would have been screaming at that point. Another personal case - last year our 4-year-old dog Maggie impaled herself on a bamboo plant stake at some point in the middle of the night. She likes to chase possums and coons on the back fence. Never heard a peep out of her until we got up at 6:30, and she never made a sound throughout the 2 surgeries at 2 separate locations across town, or for the duration of her recovery. That's my long-winded way of saying that dogs can be seriously ill and in extreme pain and will never "tell" you about it.
Cancer can also grow large enough to break through the skin and cause what looks like an ulcer or abscess. Once it has gotten to that stage, the animal may go rapidly downhill, even with the best of care.
Was Haley under a vets care?
My condolences for your loss. Its very hard, losing a beloved pet.