Jennifer Brea was 28 when she found herself unable to finish a short bike ride or run — she’d head home halfway through and sleep for hours.

“I kind of thought, ‘Oh, maybe I’m just getting older,’” says the director and star of the new documentary “Unrest.” “I was only 28, and I was like, ‘Maybe I’m just aging?’ And a part of it is because I kept going to the doctor and he’d say, ‘You’re fine, you’re fine, you’re fine.’”

She wasn’t fine — her fatigue got worse, and soon the whip-smart Harvard Ph.D. student couldn’t complete sentences or even send an e-mail. But doctors still couldn’t pinpoint anything provably wrong with her, and ultimately told her that she had “conversion disorder”: a modern-day version of hysteria.

In reality, Brea, now 35 and living in California, has chronic fatigue syndrome, or CFS — also known as myalgic encephalomyelitis. The condition causes severe exhaustion, brain fog and, often, pain. She suspects her case was triggered by a dayslong fever in 2011, although little is known about what causes chronic fatigue.

Her documentary, now showing at IFC Center, chronicles her battle with CFS, and the stigma that the little-known disease carries: Because the condition is still so under-the-radar, many believe that those with CFS are simply tired or exaggerating their symptoms.

Brea’s crusade is gaining steam, thanks in part to a handful of celebrities who’ve opened up about their own struggles with chronic, hard-to-diagnose conditions.

Singer Lady Gaga made news earlier this month when she announced that she was suffering from fibromyalgia, another chronic pain condition. She canceled a leg of a tour to recuperate — but even she faced backlash from fans who didn’t believe that her disease was real.

With both conditions, there is no blood test to confirm a diagnosis; rather, diagnoses are made based on a patient’s self-reported symptoms, such as elevated pain above and below the hips in the case of fibromyalgia.

Even though there is research into biomarkers of each condition — elevated levels of certain proteins in the blood, for example — it’s difficult to know for certain whether a person has either condition.

Another issue: Women are estimated to be six times more likely to have CFS than men and twice as likely to have fibromyalgia.

Historically, women who report symptoms such as extreme pain and fatigue are branded “hysterical,” and often treated as if their conditions are purely psychological.

Historically, women who report symptoms such as extreme pain and fatigue are branded “hysterical,” and often treated as if their conditions are purely psychological.

When Brea first got ill, she was told by a neurologist that her symptoms were likely due to forgotten emotional trauma in her past.

NYU Langone rheumotologist Paula Rackoff, MD, says that these types of reactions by the medical community are changing, partly thanks to an increase in female doctors.

“I think women respond to women differently [than men do],” says Rackoff.

But doubters are still disturbingly common.

Last week, Gaga posted about her struggles on Instagram — and defended herself against critics.

“I use the word ‘suffer’ not for pity, or attention, and have been disappointed to see people online suggest that I’m being dramatic, making this up, or playing the victim to get out of touring,” she wrote in a photo caption. “I use the word suffer not only because trauma and chronic pain have changed my life, but because they are keeping me from living a normal life.”

In her new Netflix documentary “Gaga: Five Foot Two,” streaming now, the cameras follow along as she cries, shakes and turns to a doctor for trigger-point injections, in which a doctor injects medication in tight muscles, to numb the pain radiating from her hip to her jaw.

“Do I look pathetic?” she asks in one scene as she weeps. “I’m so embarrassed.”

Gaga not only worries about her own ability to handle the pain, but also about others with the disease who don’t have the same support system — or funds for treatment — that she does.

But even Gaga stuggles to find doctors who know how to treat her still-mysterious condition. In another scene, she says she’d rather try painful treatments than sit in an MRI machine for hours, only to be told to take Advil at the end.

Brea blames the severity of her condition on how long it took her to find a doctor who believed her.

“I think there’s a good chance that if I had … been diagnosed earlier, I may not have gotten as sick as I became, and I may not have ended up in a wheelchair,” she says, adding that instead of resting initially, she pushed herself to exercise and work. Anti-viral medications and recent lifestyle changes have improved her fatigue, but she doesn’t believe she’ll ever fully “heal.”

She worries that misinformation and misdiagnoses can make it hard for patients to get support from their loved ones, too.

“I think a lot of people end up thinking it’s in their heads, and their families sometimes don’t know what to do,” says Brea. “Because they want to support the one they love, but when the expert’s saying something different, it’s very hard to know who to believe. And that lack of medical education [on the part of the doctors] contributes to families splitting apart, people getting divorced, family members being estranged.”

Brea says that while her husband has been by her side from Day 1, she still “grieve[s] for the way [her illness] has affected his dreams.”

Rackoff says that those who suspect they have either disease are perfectly justified in seeking a second or third opinion. “Don’t dismiss your gut instinct.”