She said she's got an hereditary back problem, and has been told to get lots of rest.
"She's going to rest to make sure she's well enough for the tour next year, which starts in April," her spokeswoman told us.
"She has to have some physiotherapy but the main thing is that she needs to rest. She definitely cannot dance and she is definitely not leaving the band," the spokeswoman added.
Even though Jo won't be singing and dancing, she will still do interviews and TV appearances.
Solo one day
Fans were worried when Jo revealed in August she was going to release a solo single.