Sierra would bark, then run and hide in a closet.
Herfel found the dog's behaviour worrying, especially as it matched the pain she'd been feeling in her abdomen.
She went to an emergency room and was told she had a cyst on her ovaries. However, Sierra's strange behaviour continued, so her owner decided to seek a second opinion.
This time, a gynaecologist confirmed that she had stage 3 ovarian cancer.
Herfel started chemotherapy, underwent a full hysterectomy and had her spleen removed.
Sierra wasn't done though.
In 2015 and then in 2016, she acted strangely again. Both times, Herfel's worst fears were confirmed: the cancer was back.
She ended up being diagnosed with cancer in her liver and then in her pelvic area.
Whenever a friend of hers, who also had ovarian cancer, came to visit, Sierra would hide.
"I owe my life to that dog. She's really been a godsend to me. She has never been wrong," the proud dog owner said.