In 2013, when Selena Gomez canceled shows for her world tour and decided to shy away from media, everyone though that she's having a breakdown after her big split from Justin Bieber. Others thought she had an addiction problem and was in rehab.

Unfortunately, none of these things were true: the singer was in fact, battling Lupus, a serious autoimmune disease that drastically changed her life. In an interview with Billboard, Gomez expressed her hurt over the public's speculation, adding that "I was diagnosed with lupus, and I've been through chemotherapy. That's what my break was really about. I could've had a stroke."

To her critics, "I wanted so badly to say, 'You guys have no idea. I'm in chemotherapy. You're assholes.' I locked myself away until I was confident and comfortable again," she said.

While chemotherapy is more commonly given to patients with cancer, Dr Joan Merrill, director for the Lupus Foundation of America said that the drug can help combat the disease. She told ABC News, "What [chemotherapy drugs] do is they kill any of the cells in the body that are rapidly dividing. The immune cells are rapidly dividing ... to create inflammation."

Merrill also noted that lupus patients undergoing chemotherapy don't get as high a dose as those battling with cancer, but they may also suffer the same side effects, such as nausea and diarrhea.

But what is Lupus? As defined by Mayo Clinic, it is a chronic inflammatory disease that occurs "when your body's immune system attacks your own tissues and organs." Inflammation caused by the disease can affect any part of the body systems, including joints, skin, kidneys, blood cells, brain, heart and lungs.

It can be quite difficult to diagnose lupus, as many of its symptoms mimic other ailments. However, here are some symptoms to look out for: Fatigue, fever, joint pain, stiffness, swelling, butterfly-shaped rash on face, skin lesions that worsen with sun exposure, shortness of breath, chest pain, dry eyes, headaches, confusion and memory loss.

Mayo clinic also explained that no two cases of lupus are exactly the same, making it an even more challenging disease to diagnose. The appearance of signs and symptoms may be sudden or may develop slowly over time, and most patients have flares --- when signs and symptoms get worse for awhile, but then improves or disappear for periods of time.

It is recommended to see a doctor when unexplained rashes, ongoing fever and persistent aching occur.

Read more about the disease here.