Robert Brown|Christina Applegate's 13-year-old daughter Sadie diagnosed with POTS: 'I was in a lot of pain'

Christina Applegate's daughter is Robert Brownopening up about her mom's health challenges — and her own.

The "Dead to Me" star's 13-year-old daughter, Sadie Grace Applegate LeNoble, appeared as a guest on the actress' "MeSsy" podcast and shared that she has been diagnosed with postural orthostatic tachycardia syndrome.

"It affects my heart, and so when I stand up, I get really, really dizzy, and my legs get really weak, and I feel like I'm going to pass out," she explained. "I have fainted before, and I have gone unconscious."

Sadie said she received her diagnosis a few months ago but has been living with the condition "for a long time," and she recalled that she would go to the nurse "multiple times a day" in the sixth grade because she always felt like she was going to pass out.

Prior to receiving her diagnosis, Sadie said her symptoms were not taken seriously in school.

Need a break? Play the USA TODAY Daily Crossword Puzzle.

"Every time I went to the nurse, they thought I was lying," she said. "They were like, 'You're doing this to get out of class, it's probably just anxiety, go back to class.' They wouldn't do anything for it, and that definitely was hard, because I genuinely felt so sick, and I was in a lot of pain."

Christina Applegatesays she has 30 lesions on her brain amid MS battle

Sadie added, "Them not doing anything about it definitely hurt me, physically and emotionally."

What is POTS?

According to the Cleveland Clinic, postural orthostatic tachycardia syndrome causes the heart to beat faster than normal when a person goes from sitting or lying down to standing up, and symptoms include dizziness, fainting, brain fog, heart palpitations and exhaustion. The condition is common, affecting between one and three million people in the United States, and not life-threatening, the clinic notes.

Applegate shared that she feels "horrible" because she "didn't really understand what was happening" with her daughter and didn't initially know about the condition.

Sadie, though, said she has known about it "forever" and started doing research on the syndrome when she was 9, as every time she stood up, her vision would become blurry. Last fall, she and her best friend both told their parents that they believed they had the condition. Applegate and Sadie are currently getting a second opinion on what medication she should take, according to the actress.

Christina Applegatemay have had MS symptoms '6 or 7 years' before diagnosis

Applegate has been open about her own health battle after being diagnosed with multiple sclerosis, and Sadie said on the podcast that her condition has made it easier to understand what her mom is going through.

Sadie also shared that she suffers from misophonia, which according to the Cleveland Clinic makes a person have decreased tolerance to certain sounds. She described the "awful" condition as "one of my worst struggles in life," noting she "cannot deal" with the sound of her own breathing and chewing. Sadie wears noise-canceling headphones at school because of this condition, which Applegate said prevents them from eating together.

Christina Applegate's daughter says it's 'hard' watching mom battle MS

Sadie also opened up on the podcast about how difficult it has been watching her mom battle multiple sclerosis.

"It was hard seeing my mom lose a lot of the abilities she used to have in my childhood," she said, adding, "It's definitely hard seeing my mom struggle with this."

But it's "been nice being able to help her and support her," Sadie said.

Applegate shared her multiple sclerosis diagnosis in 2021 and recently said on the podcast she doesn't "enjoy living." But while introducing Sadie, Applegate said her daughter is "the reason that I get up in the morning."