England captain Williamson feared period pain could have wrecked her Euros
The 25-year-old Arsenal defender suffers from endometriosis, a condition so painful it can lead her to collapsing onto the bathroom floor when she has a period.
In an interview with Women's Health magazine, extracts of which were published Wednesday, Williamson said: "Before the Euros I had a concussion, which they say can really impact your next period, and it was bad -- like, really bad.
"You know when you're on the bathroom floor and literally like, 'I can't move'. When it's too late to take the tablets because I'm, like, in it now."
Endometriosis is a long-term condition where tissue similar to that lining the womb grows in other parts of the body, such as the fallopian tubes and ovaries.
Period and pelvic pain, together with excessive bleeding, are among the symptoms of a condition that is also capable of leading to infertility.
Williamson recalled how stressful a bad period made her feel became before last year's game between England and Norway.
"I was like, 'It cannot happen'. Like, I actually won't be able to play," she said.
Williamson added: "I'm pretty sure if men had periods we would have figured out a way to stop them by now without doing any damage."
The Euro 2022 final victory over Germany at Wembley in July marked England's first major football trophy since the men's 1966 World Cup triumph at the London ground.
Williamson said victory had left her a fear she would spend the rest of her life chasing feelings of similar elation.
"I don't see how it could have been topped," she added.
"The only thing I'd love to do is go back and watch it from the stands. As the final whistle went, my grandma just put her head on my mum's shoulder and went, 'She did it'."
Williamson also said the high profile that came with the success of the England women's team was a double-edged sword.
"I love it because we can reach so many people," she said before adding: "I hate the fame and that side of it."
Williamson, however, explained: "If you're not prepared to (be famous as a professional women's footballer) you're doing a disservice to the game and all those women that had to pretend to be boys to be able to play.
"I understand that I stand on the shoulders of those people."