According to Elizabeth Taylor, Jackie was convinced she and JFK had an affair.

Jackie Kennedy was said to have hated actress Elizabeth Taylor.
Reason: Jackie feared that her husband and former US President John F. Kennedy would have an affair with the iconic beauty.
Astonishing details of their bitter feud are revealed in Taylor’s private diaries, meticulously kept by the late screen icon since her days as a child star.
Some of the secrets have been told to the Sunday Express by one of the late screen icon’s closest friends.
“When they are edited and published, as she requested, they will amaze fans and historians,” said the friend.
They will also blow away dozens of myths about the double Oscar winning, eight times married legend’s life. One includes how Taylor could never convince Jackie that she did not have an illicit relationship with her husband.