Despite releasing two brand new singles and embarking on a new relationship with fellow singer The Weeknd, former Disney star and singer Selena Gomez has been awfully quiet in recent months.

Now, the 25-year-old shared the reason for her seemingly sporadic public appearances and Instagram activity: she underwent a kidney transplant due to her battle with lupus.

“I’m very aware some of my fans had noticed I was laying low for part of the summer and questioning why I wasn’t promoting my new music, which I was extremely proud of,” Gomez – who is the most followed person on Instagram – told her fans.

“So I found out I needed to get a kidney transplant due to my Lupus and was recovering. It was what I needed to do for my overall health.”

The Fetish singer also explained that her donor was her longtime friend, Francia Raisa.

Francia is also a famous face, appearing on tv show The Secret Life of the American Teenager, and has been friends with Selena since they met during a visit to a children's hospital in 2007.

"There aren't words to describe how I can possibly thank my beautiful friend Francia Raisa," Selena said of her donor.

"She gave me the ultimate gift and sacrifice by donating her kidney to me. I am incredibly blessed. I love you so much sis."

Alongside the snap of her and Francia holding hands in hospital, Selena also showed off her surgery scar and images of her recovery.

The singer first revealed she had been diagnosed with lupus in an October 2015 interview with Billboard magazine, saying she was undergoing chemotherapy as a form of treatment.

She had checked herself into an Arizona rehab facility the year before, with tabloids speculating that she was addicted to drugs and alcohol or was heartbroken over her split from fellow singer Justin Bieber.

"I was diagnosed with...lupus. That's what my break was really about. I could've had a stroke," she told the magazine.

"I wanted so badly to say, 'You guys have no idea. I'm in chemotherapy. You're assholes'. But I was angry I even felt the need to say that.

"I locked myself away until I was confident and comfortable again."

LISTEN: Mia Freedman talks to Bec Craven, the woman living because of someone else's heart on No Filter.

Lupus is an autoimmune disease which causes the immune system to turn against parts of the body it is designed to protect. It can affect many parts of the body, including the skin, kidneys, heart and brain.

Those diagnosed with the disease often go through periods of sickness, and periods of wellness or remission

For Gomez, anxiety, panic attacks and depression have also been side effects of her illness.

The singer is now focused on increasing awareness and support for research for others suffering from lupus.

"Lupus continues to be very misunderstood but progress is being made," she said.

READ MORE:

• Selena Gomez opens up about her mental health struggles in AMAs acceptance speech.
• For anyone who is struggling with anxiety: this is your digital care package.
• This is what it's like to live with a chronic illness most people have never heard of.