Ms Ferrier, the MP for Rutherglen and Hamilton West, said she had experienced “mild symptoms” on Saturday and was tested for coronavirus.
However, she decided to travel by train to Westminster on Monday before getting her result because she was “feeling much better”.
She spoke for four minutes in the Commons chamber during a coronavirus debate – tweeting a video of her speech – but was told later that evening that she had tested positive for the virus.
Despite this, Ms Ferrier took a train back to Scotland on Tuesday, with SNP whips in the Commons being told about her positive test on Wednesday.
It is understood she had initially told the party she was going home because a family member was unwell.