The reign of Mary I was ironically not a period of safety for her younger sister Elizabeth. Mary constantly feared that
In the spring of 1554 what is known as Wyatts Rebellion broke out, named after one of the leaders, Thomas Wyatt, a landowner with large properties in Kent. Another one of the leaders was Henry Grey, 1st Duke of Suffolk and Jane Grey´s father, who had apparently learned nothing from the downfall of his daughter who at the beginning of the rebellion was still a prisoner in the Tower.
The motives behind the rebellion was dissatisfaction with Mary´s plans to marry a foreign prince, Philip of Spain, and while it was never an expressed goal, the implications was that the rebellion aimed to overthrow Mary and replace her with Elizabeth. At Wyatt´s trial, however, he was acquitted of any intentions to harm the Queen.
Although the rebellion was stomped out be this time in March, with both Wyatt and Grey executed, Mary had a hard time trusting her younger sister, especially since it came to light that Wyatt had written to Elizabeth. The same day as lady Jane Grey was executed, on February 12th, Elizabeth was summoned to London, it took her however over a week to get there due to illness during her journey. After lengthy interrogations, during which she assured her innocence, she was still transferred to the Tower, on this day, March 18th 1554.
Here she was confined to the Bell Tower. Considering her mother´s fate in the Tower, and the fact that her second cousin Jane had died there only the month before, one can imagine that Elizabeth feared for her life.
She was released two months later.
Source: Death and the Virgin – Chris Skidmore
Elizabeth – David Starkey