Selena Gomez ''didn't understand'' what was happening when she was first diagnosed with Lupus.

The 23-year-old singer found out she has the autoimmune disease, that occurs when the body's immune system attacks its own tissues and organs, three years ago and admits she had hoped to keep the news private.

The 'Good For You' hitmaker said: ''[It happened] three years ago... That's when I found it and I think it was just very complicated because I didn't understand and then it was something that I did want to keep private until my whole life suddenly wasn't private.''

While the starlet has had to make some changes to her lifestyle, she has come to terms with the chronic inflammatory disease and feels healthy at the moment.

She told 'The Ellen DeGeneres Show': ''You'll have it forever. It's kind of one of those situations. You just have to take care of yourself.''

Selena decided to hit back at critics on Instagram, who bashed her for apparently gaining weight while on holiday earlier this year, because she feared people thought she had ''gone off the deep end.''

She explained: ''It was the first time I'd ever been called fat... it was weird because it's not just 'Oh, she's fat.' It's like, 'She's a mess. She's gone off the deep end.' I'm like, because I'm on a vacation wearing a bathing suit? Actually technically that is a little too small for me, but I didn't care. That's so degrading.

''It's so annoying when I see it all over the place and everybody thinks they can bring me down. So my immediate response was I'm gonna post a picture and I'm like, 'I'm happy with me y'all'... I wasn't gonna let that be the story. That wasn't the story.''