Selena Gomez's new kidney ''flipped'' after she had a transplant.

The 25-year-old singer received an organ from her friend Francia Raisa in a life-saving operation over the summer, and though she woke up feeling fine after the procedure, she had an excruciating pain afterwards when she tried to nap, and had to get back on the surgeon's table.

She recalled: ''My teeth were like grinding, 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 grateful that there are people who know what to do in that situation.''

The 'Bad Liar' singer and her roommate spent the evening before the surgery enjoying an indulgent meal and having their hair French braided.

Selena explained in a joint interview on 'Today': ''I wanted us to feel good.''

Though Francia had volunteered to donate her kidney after seeing how much her pal was struggling, she admitted the idea of having the surgery was daunting, while her mother was so nervous she ''didn't want to be there'' until after she woke up from the operation.

She said: ''I had to write a will, which was scary because there's no guarantee you'll wake up.''

Though the surgeries were successful, the two pals had a long way to go in their recovery as they were only allowed to move around for an hour a day, and it was ''hard'' for them having to be so reliant on other people.

Francia said: ''It was hard because you constantly needed to ask for help.

''I think one of the most humbling experiences was needing help to put on underwear. We couldn't take showers by ourselves. It was a brutal process.''

Selena feels better than ever now, with her blood pressure and energy levels improving, her arthritis going away and having just a three to five per cent chance of Lupus returning, and she'll always be grateful to her friend.

Asked if she feels Francia had saved her life, she said: ''Because she did. That's it.

''I guess I got to the point where it was really, kind of, life or death. It's really hard to think about or even swallow.''