The 39-year-old actor and TV presenter said she had received some 'shocking news' before deleting all her social media accounts.
The actor had a much-publicised struggle after her father was diagnosed with cancer back in 2016.
Smith feared it would harm her unborn child but learnt during while filming her documentary that mental health medication can often be taken by expectant mothers.
The 'Gavin & Stacey' star revealed she tried to quit her anxiety medication and was rushed to A&E after suffering seizures.
The actor said she 'jumped at the chance' to be a part of the series, which also stars Angela Griffin, David Threlfall and Robert Glenister.
The West End veteran has been dropping some not-so-subtle hints for the last few weeks.
'Funny Girl' star Sheridan Smith returned to Instagram to pay tribute to her father, who died in 2016.
There’s no business like showbiz but it can be a tough calling, as Sheridan Smith has discovered. There aren’t many careers out there that would demand you to pile on the pounds and then ditch them again on a whim and the 35-year-old actress has admitted that she is struggling to lose weight after she “bulked up” last year for her lead role in ‘Moorside’. The star wanted to make her portrayal of Karen Matthews’ best friend Julie in the BBC drama, which is based on the shocking disappearance of Shannon Matthews in 2008, as real as possible so agreed to pile on the pounds in time for filming last year, but has admitted she’s finding it really tough to beat the bulge and she still has “a load” to lose.