It was a shocking diagnosis, but one that finally gave Felicity clarity after she had been plagued with health problems for the last 20 years.
Fatigue, fainting spells and severe migraines became the norm in Felicity's life after she came down with glandular fever as a teenager.
Her days of swimming, rowing and running came to an abrupt end as Felicity often found herself bedridden - sometimes missing school for months on end.
A fun few hours out with friends could require days of recovering, and Felicity found herself limiting her activities to conserve the little energy that she had to give.
Felicity began having bad cardiac episodes when she fell pregnant with her daughter Olivia, now two, she told Daily Mail Australia.
In one instance, when she had to go to the hospital due to severe swelling of the hands and feet, doctors noticed her heart rate was over 100 beats per minute.
They began to monitor Felicity with a blood pressure machine and decided to wait until Olivia was born before conducting further tests.
When Felicity's heart rate returned to normal after giving birth, her doctors decided there was nothing to worry about.
But Darryn wasn't convinced, and thought the Fitbit would be helpful for more than just fitness.
"My partner bought this Fitbit as I was trying to get fit but struggling with it, half to track my exercise - he wanted me to take it easy and build up gradually," she said.
"But he also definitely wanted to keep an eye on my heart rate and see what was happening."
After an electrocardiogram detected the atrial fibrillation, Felicity was immediately scheduled for a catheter ablation surgery.
During the surgery, which took place three weeks ago, a catheter was inserted into a main artery in Felicity's leg that went up to her heart.
An electrode that was at the tip of the catheter was then activated, sending radio frequency waves to burn the heart tissue that was causing the arrhythmia.
Following catheter ablation surgery, the heart is supposed to return to a normal rhythm.
And, for Felicity, at first it did.
The Kiwi felt a "huge difference" the day after her surgery.
"It was quite unsettling, because it felt very thumpy,' she said. "I'm more used to it being rapid and fluttering."
Felicity's heart rate is still spiking from time to time and she has continued to experience episodes of not feeling well.
But she said the first ablation surgery has fixed a "couple of areas", and she hopes the second will sort everything out for good.
If the surgery cannot correct Felicity's heart rate, she will be fitted with a pacemaker.
Felicity is currently trying to raise funds on GoFundMe to pay for her recent procedures, and said she is grateful that she now knows the underlying cause behind her health issues.
And she is glad that the condition has given her permission to "just stop for awhile" and "not feel like I have to keep up with everyone else".
"It made me realise I need to look after myself and take it easy," she said.
"I do have a different outlook on life. I can see this is really serious and I can focus on what is important."
