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 chat 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."