"If I offered him that breast he would completely freak out," she said. "He'd become extremely distressed and scream the house down."
Sarah is now planning a double mastectomy with immediate breast reconstruction surgery.
She had previously had a scare with a lump that turned out to be a benign cyst, so she returned to her GP and was told it was fine.
But the feeding problem continued. "Teddy just wasn't having it," she said. "I had no problems with my left breast but every time I tried with my right he would start screaming and get very upset. He wouldn't go near it."
When Teddy was eight months old, Mrs Boyle returned to her GP and asked to be referred for a scan.
She said: "I felt as if Teddy was trying to tell me something. It was what you might call a mother's instinct."
In November last year, she had an ultrasound scan at Royal Stoke University Hospital that revealed a problem related to her cyst.
She then had a biopsy, and two weeks later was diagnosed with a form of the disease that is extremely rare in young women, known as grade 2 triple negative breast cancer. She said: "My mind went into meltdown.
"But I felt so proud of Teddy for bringing the problem to my attention. The cancer inside the cyst had been growing for three months - the exact same time Teddy stopped feeding from my breast."
The day she was diagnosed she was told to stop breastfeeding so she could start chemotherapy. She is halfway through the treatment.
She said: "It was sad losing my hair but I've been rocking lots of different wigs and am comfortable being bald around my loved ones now.
"I never imagined that I would be diagnosed with cancer so young but the hospital have been fantastic.
"I want other women to be aware of any lumps in their breasts and to always get them checked out."
