When Chandra Wilson exclusively opened up to PEOPLE about her daughter Sarina McFarlane’s rare vomiting disorder, the Grey’s Anatomy star willingly shared one of the most terrifying side effects she witnessed.

“When first started cycling, she weighed 100 lbs. and then she went down to about 70 lbs.,” reveals Wilson, 47. “That was scary to watch. Then, once the meds were introduced, she went completely in the other direction. There’s no rhyme or reason to it. It’s the strangest illness, but it can be incredibly debilitating.”

At 16 years old, McFarlane began experiencing symptoms similar to that of food poisoning. However, as the symptoms became repetitive and more painful, questions began to arise.

“She would get these terrible bouts of vomiting and stabbing abdominal pains,” Wilson tells PEOPLE in this week’s issue. “I thought, ‘This was crazy.’ Something was wrong with my daughter, and nobody could tell me what it was.”

McFarlane admits she was “scared, frustrated and depressed” during the months leading to her diagnosis.

“People in high school thought I was throwing up because I was trying to lose weight,” she says.

After 10 frustrating months of unanswered questions and endless hospital visits due to dehydration, McFarlane was finally diagnosed with mitochondrial dysfunction (severe depletion of the body’s cellular energy supply) and cyclic vomiting syndrome (CVS), a rare disorder that presents with random attacks of committing, nausea, and extreme exhaustion, in 2010.

Now McFarlane is 23, and Wilson says her role has shifted a bit when it comes to her daughter’s care.

“My job as an advocate now that she’s an adult, is just to kind of double check in on the medical protocol to make sure she’s okay and I follow her lead with it,” she says. “I don’t want to be the person where every time she calls I’m like, ‘Did you take your meds?’ That would just be annoying, but I do kind of do that!”

Though McFarlane, who is studying screenwriting at Cal State Northridge, says she is no longer allowing her diagnosis to negatively affect her life, she admits that there were times she just couldn’t help it.

“When it comes to school, it can throw me off in a really horrible way,” McFarlane admits. “There was one semester where I had to take every class over because I missed so much. Last semester, I just didn’t let that happen. Every time I woke up in the hospital and I felt okay, I wrote a script. Every time I was awake, I studied for another class. I just made sure that I studied.”

“I could be sad about it,” she says, “but it’s going to come back anyway.”

Grey’s Anatomy airs Thursdays (8 p.m. ET) on ABC.