Her GP sent for a mammogram and ultrasound. The results of those tests got her sent to a specialist for a biopsy. Again, the specialist assured her.
But it came back cancerous.
Nine woman a day - about 3300 a year - are diagnosed with breast cancer in New Zealand, according to the Breast Cancer Foundation NZ.
Of those, one will be younger than 45 - roughly one a day or 365 a year.
More than 600 Kiwi women die of breast cancer every year.
"You're stunned, but you think, okay, what do we do now? Have we caught it early? What can we do about it?
"I'll never forget, a friend rang me and said, 'I heard you got some great news.'
"I was taken aback, but he said, 'You found it, mate, and that is damn good news because now you can get rid of it.' "
"We talked to the boys right from the initial diagnosis. We used the word cancer. We didn't hide that.
A surgeon removed the lump, but McDiarmid sensed something wasn't right.
The cancer was a lot worse than anyone thought.
"My surgeon came in to see how I was and I could tell from his face that he wasn't happy."
The results showed it had spread to her lymph nodes and she needed to have a double mastectomy along with chemotherapy and radiation.
"I wanted them off right from the beginning. They threatened my life - I don't need them, I don't want them," McDiarmid said.
She chose not to get reconstruction surgery.
"My breasts had done their job, they had fed my three children. They did not define me."
Two years later, she has finished treatment and is focused on keeping healthy.
McDiarmid's petite frame no longer has the weight of F-cup breasts. Running is a lot easier and her goal is a half marathon.
There's another bonus.
"The kids say, 'Mum, you can mow the lawn like Dad now - with your shirt off!' Which always has me laughing."
When the opportunity to be part of a Breast Cancer Foundation NZ campaign came up, McDiarmid put her name forward.
October is Breast Cancer Month, and this year's theme is, "Knowledge Beats Breast Cancer."
The foundation wants women to be breast aware from age 20 - checking, feeling, keeping an eye on any changes and going to the doctor if something doesn't feel right.
McDiarmid and two other breast cancer survivors were photographed in a Survivors Collection, replicating masterpieces by painters Rubens and Rembrandt.
In the original paintings, the women's breasts are recognised today to show signs of breast cancer. There were signs of dimpling, puckering, lumps and marks that today are recognised as breast cancer.
"While the hidden danger within the original masterpieces is tragic, we also wanted to show that with the right knowledge and awareness, breast cancer can be overcome," Breast Cancer Foundation NZ chief executive Evangelia Henderson said.
The campaign celebrates the improvement in medical care in the 21st century while reminding modern women to "know your normal".
"Just look at how far we've come in being able to treat it when you compare the tragedy of the original masterpieces to the brave women who feature in our recreations. Hopefully, they will serve as a reminder to us all that we should stay breast aware."
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