The singer-and-actress - who has been dating 17-year-old `Baby` hitmaker Justin Bieber since the beginning of this year - reportedly felt ill while taping an interview with chat show host Jay Leno and was taken to a Los Angeles hospital immediately afterwards.
Sources told gossip website TMZ, Selena, 18 was complaining of nausea and a severe headache and is currently undergoing tests.
The exact cause of her illness is not yet known.
Meanwhile, Selena revealed during her chart with Jay on `The Tonight Show`that her parents, Mandy and Rocardo - who divorced when she was five - are huge fans of her boyfriend.
She said: "He passed the test."
Justin`s family also approve of Selena after he took her on a recent trip to his hometown of Stratford, Ontario in Canada to meet his family.
A source said: "Justin and Selena had lunch with Justin`s cousins and his aunt and uncle. They were really sweet and make an adorable couple.
"They looked happy and in love. They acted just like everyday people, very down-to-earth. Justin is part of the community, he`s one of us and is as friendly as can be."