In December 2013, Gomez canceled the Asian and Australian legs of her tour, fueling speculation about her health.
“Selena has been going full throttle the past few years and her lupus is really catching up with her right now,” said a source. “She knows that she needs to take some time to address the disease and look after herself better if she wants to live a full and healthy life."
Lupus is a mysterious autoimmune disease where the body's immune system attacks normal, healthy tissue. Symptoms include joint pain, facial swelling, fatigue, inflammation and headaches.
It's unclear what exactly causes lupus, and there is no cure, but eating well and getting lots of rest is crucial, according to the book Lupus and Me.
Last week, rumors surfaced that Gomez, 21, was taking time off from her tour because of a nervous breakdown, but her camp denied those reports.
"Selena is not having a breakdown," said a source. "She wants to be the healthiest that she can be and this means taking a little break. But she is fine. She is OK."
Meanwhile, a tabloid report recently claimed that "Breaking Bad" star Anna Gunn is battling lupus, and attributed her shocking recent weight loss to the condition.
Earlier this month, actress Kristen Johnston revealed she was diagnosed with lupus myelitis and spent two weeks being treated for the illness at the Mayo Clinic.
Johnston isn't the celebrity who has gone public about coping with lupus. In 2010, Lady Gaga said she tested "borderline positive" for lupus. Gaga said she has a genetic predisposition toward lupus, and said her aunt died from it.
In 2012, singer Toni Braxton was hospitalized for health complications related to lupus-induced blood clots. She is doing fine now.
In 2011 and again in 2012, Nick Cannon was hospitalized for kidney failure induced by lupus nephritis. Cannon is now doing better after scaling back his hectic work schedule.