She had taken some over-the-counter cures for a cold, and they reacted with other medication in her system, a source tells PEOPLE. She has been in the London Clinic since she was admitted on Sunday.
It's a familiar place for the singer, 26. Her life has been checkered with health issues – she even came close to death, her father has said. But this time it's different, with Winehouse feeling healthier and more self-confident.
The singer's father, Mitch Winehouse, last month spoke about her recovery, saying that she had been "healthy for a year." And he has also pronounced himself happy with her new look, saying her recent boob job was "great."
The source agrees, saying the plastic surgery has helped Winehouse feel "really good about herself ... She's been saying how she wants to get back in the gym."