The news came as a complete shock to her fans, and fellow celebrities, of whom wished her all the best.
So how is she doing?
"Selena is resting," a source told Us Weekly. "It had been a couple tough weeks leading up to this. She is definitely improving every day." In the meantime, the singer is focusing on friends and family. "She wants to better herself every day. That's her priority."
According to People, Selena was sent to hospital twice in a matter of a couple weeks, due to low white blood cell count. During her second visit, she had a panic attack which is what eventually led her to seek treatment.
"She has had a tough few weeks and the panic attack in the hospital was the tipping point," a source told People.
Thankfully she is doing a lot better now.