That initial spark of discomfort grew to become a pattern of injuries which led to unresolved pain which caused her to limp.
Physiotherapy offered little relief so her mother took her for blood tests, X-rays and an MRI scan.
The tests saw Lily diagnosed with juvenile idiopathic arthritis.
Then began the long journey to find medications which would help prevent further damage and ease the symptoms.
But the condition became progressively worse - the pains in her hip were joined by pain and stiffness in her knees.
Lily said the simple tasks of putting on shoes or walking up some stairs became "real challenges".
Her after-school activities had to be cut back. Ballet was the toughest to let go, as she had danced with the same girls for more than five years.
By 15, she was being treated with small doses of chemotherapy but the side effects outweighed the benefits so new treatments were devised.
"Because I quickly became affected top-to-toe, finger-to-finger, and developed fibromyalgia, another form of arthritis, I had to alter my lifestyle and plans for the future," she said.
That "altered lifestyle" now includes pacing herself, gentle exercise, and injecting an immuno-suppressant drug every other week.
She said a healthier diet had also proven beneficial.
Staying positive was also important, and she said that was where Arthritis New Zealand had come in.
"I've attended several helpful seminars and workshops locally and also attended a conference in Wellington and a camp in Auckland for youth with arthritis, which were both educational and fun," she said.
Arthritis New Zealand also saw her make the stage in December 2014.
Along with a handful of fellow young people living with arthritis, she had the thrill of performing in Orange, an original musical theatre show written and directed by Geraldine Brophy.
"It showcased a metaphorical journey to happiness and contentment as a literal one.
"It was great to be immersed in a creative work setting with young women like me, as well as professional actors and crew."
An Arthritis New Zealand spokeswoman said the condition was the leading cause of disability in the country and there were nearly 18,000 people receiving sickness or invalid benefits as a result of musculoskeletal conditions, the bulk of which were arthritis.-The annual appeal will run through until next Monday with street collectors. To donate, visit: arthritis.org.nz