The Arsenal defender suffers from endometriosis which can make her periods so debilitating that she is left lying in pain on the bathroom floor.
Williamson, 25, told Women’s Health magazine: “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 the lining of the womb grows in other places, such as the ovaries and fallopian tubes.
Painful periods, pelvic pain and cramping, uncomfortable bowel movements and excessive bleeding are among the various symptoms. Infertility is also possible.
Williamson recalled how stressful the threat of a bad period became before last year’s England v Norway game.
She said: “I was like, ‘It cannot happen’. Like, I actually won’t be able to play.
“(Having an endometriosis flare-up) is a big fear when you get to a tournament not injured. I don’t change too much around (my cycle) now.
“Unless I’m on the floor. And then I’m like, ‘I won’t make it today’.
“I’m a professional athlete, I’ve always been like, ‘Let’s get on with it’. But you get to a certain age when you actually say, ‘This is a really big f****** problem’.
“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 marked England’s first major football trophy since the men’s 1966 World Cup triumph.
Williamson described the emotional comedown from winning the Euros as “hard”, adding: “It scares the s*** out of me if I’m honest that every day for the rest of my life I will chase that feeling.
“I don’t see how it could have been topped. 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’.”
She also described her mixed feelings about now having celebrity status.
Williamson said: “I love it because we can reach so many people,” but added: “I hate the fame and that side of it.”
She added: “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.”