The impact of sportswomen's menstrual cycle on performance is "the last taboo" in sport, says former British tennis number one Annabel Croft.
Croft told BBC Radio 5 live that women "suffer in silence" on the subject.
Her comments were made following British number one Heather Watson's Australian Open first-round defeat, in which she admitted to feeling unwell.
Watson told BBC Sport: "I think it's just one of these things that I have, girl things."
The 22-year-old said she suffered from dizziness, nausea, low energy levels and spells of feeling light-headed as she lost 6-4 6-0 to Bulgaria's world number 52 Tsvetana Pironkova.
She called for the doctor towards the end of the first set and was visibly struggling as Pironkova won the last seven games.
After the defeat, Watson said she was confident it was not a return of the glandular fever she suffered in 2013.
"I get it sometimes," she said. "I'm going to go and see the doctor afterwards and see if there's anything I can do to help with times like these in the future."
Croft described Watson's comments as "brave" and that women would "identify completely" with her symptoms.
She said: "Women's monthly issues seems to be one of those subjects that gets swept under the carpet and is a big secret.
"Women dealing with these issues at any time is hard enough, but actually trying to go out there and trying to play top-level sport at one of the most crucial times in the calendar year. It is just really unlucky.
"I think women do suffer in silence on this subject. It has always been a taboo subject."
Watson went into the first Grand Slam of the season in Melbourne fresh from winning her second WTA title in Hobart last week and at a career-high ranking of 38.
"It's really frustrating, especially at the one time I really do want all my energy and to be 100%," said Watson.
"But it happens and you're dealt with different cards on different days and I should have dealt with it better. It's a real shame and it sucks."