Lupus can harm the skin, kidneys, heart, nervous system and blood cells.
Gomez was reportedly diagnosed with the disease a couple of years ago and she has suffered multiple "flares" recently that cause facial swelling, extreme fatigue, headaches and joint pain.
"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," the source added.
The Come & Get It singer has axed 13 shows from the Asian and Australian leg of her tour.