The Lioness, who led her side to glory in the European Championships last year, is training to be an accountant, having decided to pursue it as a potential back-up career.
"Originally, when I started it, women's football wasn't as secure as it is now," she said.
"I started it as a bit of an insurance policy for me to take the pressure off football - so if it didn't work out, I have something else."
The rupture to her anterior cruciate ligament and subsequent surgery ruled her out of this summer's World Cup in Australia and New Zealand, which runs from 20 July to 20 August.
Williamson revealed she'd had a "physical or mental question mark" hanging over her every day since last October, having played huge amounts of football for both England and Arsenal.
"I have to listen to my body, give it what it needs," the defender added.
'A secure thing to have in my locker'
Since undergoing surgery, Williamson has put her interest in finance into action by investing in Toca Football, which operates indoor football centres and themed entertainment venues.
It is her first business venture, and she hopes it will stand her in good stead for a post-football career.
"I enjoyed maths at school, so it seemed like a secure thing to tick off and that I always have in my locker," she said.
"I don't want to wish away my career, but it is a short-lived time for all of us."