Lady Gaga says she suffers from fibromyalgia; what is it?

Days after debuting her Netflix documentary Lady Gaga: Five Foot Two at the Toronto Film Festival on Friday, the singer/actress says she wants to connect with others struggling with fibromyalgia.

"In our documentary the #chronicillness #chronicpain I deal w/ is #Fibromyalgia I wish to help raise awareness & connect people who have it," she tweeted Tuesday.

She also replied to a user who commented on how Gaga's discussion of the illness opened the door for others to talk about their experiences with chronic pain.

"I am praying that more and more people come forward and we can all share what helps/hurts so we can help each other," she wrote.

Other celebrities with fibromyalgia include Morgan Freeman, Sinéad O'Connor and Janeane Garofalo.

So what is it?

It's a musculoskeletal illness that causes pain across the body. The disorder can also impact the afflicted person's energy level, sleep and mood, the Mayo Clinic says. About 4 million people in the U.S. deal with the disease, according to the Center for Disease Control and Prevention (CDC), and most sufferers are female.

While the cause of fibromyalgia still remains a mystery, medical experts believe genes could play a role, the National Fibromyalgia Association reports. Stressful events in the person's life, such as physical trauma or surgery (doctors repaired a tear in her hip in 2013), might also correlate to their chronic pain.

Another possible cause? The patient's immune system.

"We're hoping some day we'll be able to say exactly how your immune system is causing damage to the sensory nerves that results in fibromyalgia pain," rheumatologist Richard Chou told USA TODAY in 2013.

In making a diagnosis, the Mayo Clinic says doctors check to see whether the patient has exhibited symptoms for at least three months and attempt to rule out other causes since pain, headaches and fatigue are often present in other maladies.

Treatment options include aerobic exercise, cognitive behavioral therapy and pain medication (prescribed or over-the-counter), according to the CDC.

Gaga herself has posted about ways she manages her chronic pain. She wrote last year that she often uses saunas, blankets that trap heat and ice baths to soothe her muscles.

Gaga also acknowledged that what works for her might not work for all fellow sufferers.

"Everyone's body and condition is different," she said, recommending that patients ask their doctor about treatment as well.

Gaga: Five Foot Two will be available to stream on Netflix Sept. 22.