Amelle Berrabah broke down after receiving hand-delivered "hate mail".
A friend of Amelle's revealed: "She moved back to her mum's house, but she barely went out. She was getting hand-delivered hate mail and her Facebook account was hacked by someone saying they were a fan of Keisha.
"She's been getting loads of nasty texts too. It really affected her. She was scared to leave the house in case she was attacked. She looks really skinny, and hasn't worked with the band since they came back from Los Angeles."
Last week, Amelle's bandmates Jade Ewen and Heidi Range revealed the 25-year-old singer will be taking a three-week break from the band to stay at a private health facility after recent stresses within the band - which saw original member Keisha replaced by Jade - became unmanageable.
The source continued: "Heidi and Jade are very supportive of Amelle. Jade was bullied when she was younger, so she understands. Both Jade and Heidi just want Amelle to feel better."
Promotional duties will still be carried out by Heidi and Jade in Amelle's absence, but planned performances, including the new line-up's debut in Leeds last Sunday (18.10.09), have been shelved.