Kelly Osbourne may have epilepsy.
The 'Fashion Police' host - who was rushed to Cedars Sinai hospital in Los Angeles after suffering a 30 second seizure on the set of the E! series on Thursday - has been warned by doctors that the dramatic health scare could be much more serious than they initially suspected.
A source told gossip website TMZ.com that Kelly's medical team are running tests and she will remain in hospital for at least a few more days while they try to determine what caused the sudden seizure.
The insider said she is currently on anti-seizure medication and doctors are closely monitoring her as they believe the seizure was extremely serious.
The 28-year-old star took to her Twitter account on Friday to show her appreciation for all the "beautiful well wishes" she received following the frightening incident, which saw her rushed to hospital after she had a seizure while filming alongside Joan and Melissa Rivers.
Kelly posted a picture of herself in hospital and wrote: "Thank U all 4 UR beautiful well wishes yes I did have a seizure they are just trying to figure out why? (sic)"
A medical emergency worker who was in the audience rushed to help her, and she was later shielded with blankets as she was stretchered into the ambulance.
Kelly - the daughter of Black Sabbath rocker Ozzy Osbourne and TV presenter Sharon Osbourne - is said to have turned to her co-host to say she didn't feel well before falling out of her chair shaking.
Sharon also expressed her gratitude to well wishers on Twitter, writing: "I want to thank everyone for their concern about @MissKellyO. She's doing fine she's at Cedars and is surrounded by the best doctors & love."