The singer - who is in a relationship with Manchester City soccer player Wayne Bridge, 31 - has admitted herself into an unknown facility for "unconfirmed reasons".
However, Frankie's spokesman is adamant she is not receiving treatment for alcohol or drug related issues.
She wrote on her page: "Sorry I haven't tweeted in a while. I've not been very well. Miss u @MollieTheSats @unahealy @vanessawhite @RochelleTheSats I'll b back soon x (sic)"
Frankie missed several band commitments in the last week, including a visit to a new Marie Curie Hospice Centre being built in Solihull, West Midlands, England.
The brunette star also skipped a group session with their vocal coach Priscilla Jones.