According to the author of the book, the girl was wearing a special colorful robe because she was a princess and hadn't married yet. After something bad happened, a servant made her leave the room and locked the door behind her.
2 Samuel 13:18: And [she had] a garment of divers colours upon her: for with such robes were the king's daughters [that were] virgins apparelled. Then his servant brought her out, and bolted the door after her.
This verse mentions that a woman should not wear that which pertains to a man, yet in 2 Samuel 13:18, Tamar is described wearing a garment traditionally for king's daughters.
Deuteronomy 22:5: The woman shall not wear that which pertaineth unto a man, neither shall a man put on a woman's garment: for all that do so [are] abomination unto the LORD thy God.