The 29-year-old opened up about the terrible symptoms she experienced throughout her teens and twenties in her eighth Lenny newsletter.
She described how nonstop vaginal bleeding, severe cramps, exhaustion and mood swings were often misdiagnosed, and she was usually told she had some kind of digestive problem or appendicitis.
The disorder, in which the lining of the uterus grows in other parts of the body, affects around 10% of women, with half of those affected often being found infertile.
But despite being incredibly painful and debilitating, not much is known about this relatively common disorder.
Lena hopes to change this, and wants other girls to get properly diagnosed so they can find a solution to the condition.
In her newsletter, Lena describes how constantly being misdiagnosed caused her to lose ‘all trust in or connection to’ her own body.
She explains from her first period how things weren’t right.
‘The stomachaches began quickly and were more severe than the mild-irritant cramps seemed to be for the blonde women in pink-hued Midol commercials,’ she wrote.
'Those might as well have been ads for yogurt or the ocean, that's how little they conveyed my experience of menstruating.'
She continued: 'During the worst of it, my father brought me to the ER, where they prodded my appendix and suggested it might be food poisoning and that we should go home and wait it out. My mother placed a pillow under my lower back, and I moaned in the guest room, where no one could hear me, my legs spread like a woman in labor.'
'When I was 16, the sporty, tan gynecologist asked no more than five questions before putting me on birth control,' Lena wrote, adding: 'Easily swayed by warning labels and lists of potential side effects, I swore the pill worsened my anxiety.'
With the help of the right doctor however, who she only discovered while filming the first season of Girls, Lena was finally diagnosed correctly.
'I saw his eyes flicker as he began to make clear connections: between the irregular periods and the crippling stomach pain, the chronic exhaustion and the intense shifts in mood around my period. The urinary-tract pain even when tests indicated nothing was wrong. The weight fluctuations....
'"Let me guess," he said. "You've had your appendix poked at a lot." I laughed with relief before I even heard him utter the word endometriosis.'
Read the full version of Lena’s newsletter here.