Endometriosis occurs when tissue similar to the womb lining grows in other places, usually within the pelvic cavity.
Williamson led England to Euros glory in July after they defeated Germany in the final to claim their first-ever major title in front of a record crowd on home soil.
The 25-year-old, who was diagnosed with endometriosis last year, said she was concerned about the condition flaring up before England's semi-final against Sweden, which they went on to win 4-0.
"I was like, 'it cannot happen'. Like, I actually won't be able to play," Williamson said in an interview with Women's Health magazine.
She added that 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 menstrual cycle) now. Unless I'm on the floor. And then I'm like: 'I won't make it today'," she added.
"Before Euros I had a concussion, which they say can really impact your next period, and it was 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."
When asked how she has dealt with the difficult periods that characterise the condition, the Arsenal defender said: "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 fucking 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."
