וַיִּרְאֶהָ יְהוּדָה, וַיַּחְשְׁבֶהָ לְזוֹנָה: כִּי כִסְּתָה, פָּנֶיהָ.
When Judah saw her, he thought her to be a harlot; for she had covered her face.
מהר"א אשכנזי writes that it is the practice in Islamic countries to this day (when מהר"א אשכנזי was writing) to brand a letter on the forehead of harlots, and so harlots would typically cover their faces.
He explains along these lines that in Bereishit 38:24
וַיְהִי כְּמִשְׁלֹשׁ חֳדָשִׁים, וַיֻּגַּד לִיהוּדָה לֵאמֹר זָנְתָה תָּמָר כַּלָּתֶךָ, וְגַם הִנֵּה הָרָה, לִזְנוּנִים; וַיֹּאמֶר יְהוּדָה, הוֹצִיאוּהָ וְתִשָּׂרֵף.
"And it came to pass about three months after, that it was told Judah, saying: 'Tamar thy daughter-in-law hath played the harlot; and moreover, behold, she is with child by harlotry.' And Judah said: 'Bring her forth, and let her be burnt.'"
הוֹצִיאוּהָ וְתִשָּׂרֵף means take her out and brand her forehead.