Selena Gomez needed to have further surgery after her kidney transplant because the donated organ starting turning around inside her body.

The Kill Em With Kindness singer revealed on Instagram in September (17) her best friend Francia Raisa had donated her kidney to her and they had undergone the transplant surgery over the summer.

During an emotional interview with the Today show on Monday (30Oct17), Selena confessed that the surgery wasn't straightforward for her as she started experiencing pain after coming out of the operating theater.

"I started to attempt to fall asleep, and in the middle of that process, I started hyperventilating and there was so much pain," she said. "I was freaking out. It was a six-hour surgery that they had to do on me, and the normal kidney process is actually two hours... Apparently one of the arteries had flipped. I'm very thankful that there are people who know what to do in that situation."

The 25-year-old became seriously ill over the summer as her kidneys began failing due to her battle with autoimmune disease lupus.

"I had arthritis. My kidneys were shutting down. My mentality was just to keep going," she said, while wiping away tears.

She didn't want to ask anyone to be her donor, but after seeing her being so ill, her roommate Francia volunteered.

"One day she came home, and she was emotional. I hadn't asked anything. I knew that she hadn't been feeling well," Francia said. "She couldn't open a water bottle one day and she chucked it and just started crying.

"And she goes, 'I don't know what to do. The (waiting) list is seven to ten years long. And it just vomited out of me. I was like, 'Of course I'll get tested.'"

Francia underwent a series of tests within a day, a process that usually takes months, and they turned out to be a match. She admitted she did worry about how the operation would affect her health and that she had to write a will as a precaution.

Since the operation, Selena has noticed a big difference in her health and credits Francia, 29, with saving her life.

"As soon as I got the kidney transplant, my arthritis went away," she said. "My lupus - there's about a three-five per cent chance it'll ever come back. My blood pressure is better. My energy, my life has been better."