‘And Tamar was told saying, “Look, your father-in-law goes up to Timnah to shear his sheep.'
It would appear that Tamar was not invited to the funeral and mourning for her mother-in-law and realises that she has been ostracised. He has completely ignored her. This brings home to her that he has no intention of carrying out his promise in giving her to Shelah, for she knows that Shelah is now of age. So when she hears that he is coming to nearby Timnah she decides to act.