She ignored the symptoms for three days and even left for a family holiday.
But as her condition worsened she had to visit doctors – who quickly confirmed that she was suffering from a stroke ... even though she was just 18.
Two years on, Millie is studying history of art and Spanish at Edinburgh University – but there have been lasting effects.
Now, though, she is focusing on raising awareness of stroke in young people.
Millie, now 20, started to feel ill in July 2013, just after finishing school.
“I had no idea it was a stroke and had not been feeling ill beforehand,” she said. “I suffered migraines so thought that was the cause. I felt drowsy, couldn’t get my words out and was droopy on one side of my face, but I still thought it was just a really bad headache.”
Not suspecting it could be something more serious, Millie and her family left for a holiday in the Channel Islands.
But after the teenager spent most of the early part of the visit to Guernsey sleeping, her dad took her to hospital.
To her family’s surprise, CT and MRI scans revealed she had suffered a crippling stroke.
“It was bizarre,” said Millie. “I didn’t really register at first, but my dad was very worried, it’s not something you would expect to happen to your teenage daughter.”
Tests revealed an undiagnosed hole in Millie’s heart had caused the stroke and she had a successful operation last year to treat the condition.
She now takes aspirin every day, but because of the delay in getting treatment, she has some long-term difficulties.
She said: “I still have some speech problems and I still get quite tired. I also have anxiety.
“It’s been a long process but luckily because I’m young my brain has been able to re-wire itself in a way it might not have with an older person”
As well as starting her degree, Millie has also raised money and awareness for the Stroke Association in Scotland.
Last year she hitchhiked from Edinburgh to Paris with a university friend to collect cash for the charity, which is holding a fundraising run in Cramond on March 6.
“I want to help raise awareness and let people know that strokes can happen when you are young,” Millie said.
“I didn’t realise I had a stroke for the first couple of days because I didn’t think it would happen to me.”
Kirsty Scott, community and events fundraising manager at the Stroke Association, said: “We’re encouraging people in Edinburgh to take part in our Resolution Run. You can choose your own pace and distance.”
To find out more, see www.stroke.org.uk/resolution