וַתָּ֣קָם וַתֵּ֔לֶךְ וַתָּ֥סַר צְעִיפָ֖הּ מֵעָלֶ֑יהָ וַתִּלְבַּ֖שׁ בִּגְדֵ֥י אַלְמְנוּתָֽהּ׃
她起身離去，除下面紗，又穿上她守寡的衣服。
Sforno
ותלבש בגדי אלמנותה, because she was no longer willing to be married, seeing that she had achieved her purpose and was with child from the man whose seed she wanted to perpetuate.
Radak
ותלבש, her conduct proves that she had no intention of becoming pregnant by anyone other than Yehudah. Many women of that period had an overpowering desire to somehow be impregnated by the sperm of Avraham the patriarch, even though it might have been diluted with other genes. Being biologically connected to Avraham was an important status symbol.