Jade Goody was rushed to hospital after collapsing following chemotherapy.
Jade, 27, had discharged herself from hospital but had to return after becoming violently ill and collapsing, The Sun reports.
A source said: "Jade has gone through an awful few days and has been very ill. She collapsed at the end of the week and had to go back into hospital.
"She is a fighter, but the treatment has just taken everything out of her and she has been in excruciating pain."
Jade's poor health means she may not be well enough to attend a "welcome home" party for her partner Jack this week in central London.
Jade's spokesman Max Clifford said: "Jade has been in a lot of pain and she is going through hell.
"It is a battle for life, but her condition has not worsened and treatment is ongoing."