Genèse 38:15

And Jehuda saw her; but she seemed in his face as an harlot, because she had provoked him to anger in his house, and Jehuda did not love her.

AND HE THOUGHT HER TO BE A HARLOT. This was because she was sitting at the cross-roads. For she had covered her face, and he could not see her. A Midrash of our Rabbis explains: For she had covered her face, i.e., that when she stayed in Judah’s house, she had acted modestly, always covering her face, and therefore he did not suspect her. This is Rashi’s language.
Now the Rabbi’s [Rashi’s]. literal interpretation is feasible since it was the way of the harlot to sit at the cross-roads, just as it is written, And she sitteth at the door of her house, on a seat in the high places of the city, to call to them that pass by, etc. Accordingly, the verse states that because her face was veiled he did not recognize her. But according to the Midrasn of our Rabbis which states that she covered her face in her father-in-law’s house, meaning that she hid herself from him while being in his house and that he never saw her face, how would he recognize her even if she were not veiled?
It further appears to me to be correct, in line with the literal sense of Scripture, that the verse is stating that he thought her to be a harlot because her face was veiled, since afterwards it states, For he knew not that she was his daughter-in-law. The reason for the covering of the face is that it was the way of the harlot to sit at the cross-roads wrapped up in a veil, with part of the face and hair uncovered, gesticulating with the eyes and lips, and baring the front of the throat and neck. Now since she would speak to the by-passer in an impudent manner, catching him and kissing him, she therefore veiled part of the face. Furthermore, harlots sitting by the roadside veil their faces because they commit harlotry even with relatives. Sodomites still do it to this day in our countries, and when they return to the city they remain anonymous.
Thus we have learned in a Mishnah: “There are three kinds of head-nets: that of a girl, which is susceptible to midras or Zavah — (see Leviticus 15:2-6; 25-26) — to an object which is used as a seat. An object not so used, but which serves as a garment or a container, is susceptible only to corpse-uncleanness (see Numbers 19:14-17). If it serves none of these purposes, it is not susceptible to any uncleanness. uncleanness; that of an old woman, which is susceptible to the uncleanness of a corpse, while that of a yotza’ath chutz, [literally, ‘she who goes outside’], is not susceptible to any uncleanness.” Now a yotzath chutz refers to the harlot, the nafkat bro of Onkelos (she who goes outside)., who places the head-net on part of the head. It does not serve her the purpose of lying on it, for in that case it would be susceptible to midras-uncleanness. or Zavah — (see Leviticus 15:2-6; 25-26) — to an object which is used as a seat. An object not so used, but which serves as a garment or a container, is susceptible only to corpse-uncleanness (see Numbers 19:14-17). If it serves none of these purposes, it is not susceptible to any uncleanness. Nor does she cover her head with it, for in that case it would be susceptible to corpse-uncleanness. Instead, she uses it to dress up the ends of her hair, in order that it be partly visible from beneath the net, and this is why it is not susceptible to any uncleanness.

כי כסתה פניה, only at this point, in accordance with our commentary on the word ותתעלף, (verse 14)

ויראה....כי כסתה פניה, so that he did not recognise her. This is why he considered her a harlot, seeing that she had positioned herself so prominently at the crossroads. Had she not covered her face, he would have recognised her as his daughter-in-law and would not have slept with her. Our sages in Sotah 10 say that on the contrary, she used to keep her face covered in her father-in-law’s house and was extremely chaste. When he now saw a woman sitting at the crossroads, the last thing he would think of was that it was his daughter-in-law Tamar.

ויחשביה לזונה, “he considered her as a harlot.” Why should Yehudah have cared? He turned around to resume on his way to the shearing. However, Tamar raised her face to heaven to pray and ask G–d why she should not be able to give birth to a worthwhile and intelligent son from the semen of such a righteous man as Yehudah. Upon hearing her prayer, G–d immediately dispatched the angel Michael who made Yehudah have second thoughts and turn around. The Torah here wrote: ויט אליה, “he turned to her,” and we find the same expression in Numbers 22,33 where it is written of Bileam’s she-ass: ותט לפני, “she turned around before me;” (the angel speaking to Bileam) in both instances the angel Michael was the subject. (subject discussed in B’reshit Rabbah 85,8)