Is it permissible for a woman to bathe or shower after she immerses in a Mikveh, in order to clean herself from the water of the Mikveh?
This question was already addressed by the Rishonim (Medieval Halachic scholars), including the Mordechi (Germany, 1240-1298), in Masechet Shavuot (chapter 2, 750), and others.
The Or Zarua (Rabbi Yishak of Vienna, late 12th-early 13th century) contended that a woman should not bathe after immersion, on the basis of a Halacha mentioned in Masechet Shabbat (14) concerning immersion for the purpose of eating Teruma. The Sages enacted a provision that one who immerses so that he can partake of Teruma (which requires a state of ritual purity) may not bathe afterward. One who sees a person bathe and then partake of Teruma may mistakenly think that it was the bath, rather than the immersion in the Mikveh, that purified the individual and enabled him to eat Teruma. People might then come under the impression that to achieve Tahara (ritual purification), one needs simply to take a bath or shower, without having to immerse in a Mikveh. This erroneous conclusion would result in many people eating Teruma in a state of impurity, which constitutes a grave Halachic infraction. The Sages therefore enacted that one should not bathe after immersing in the Mikveh for the purpose of eating Teruma.
The Or Zarua extended this provision to the case of a woman who immerses in the Mikveh after her period of Nida. Here, too, people might mistakenly conclude that the bath or shower is what divests the woman of her state of impurity and allows her to reunite with her husband. Therefore, the Or Zarua contends, a woman should not bathe or shower after immersion, so that people realize that it is the Mikveh, and not the bath or shower, which renders her Tehora (pure).
The Rama (Rabbi Moshe Isserles of Cracow, 1520-1572) codifies this ruling, noting that this was indeed the prevalent custom at his time (Yore De’a 201:75). Ashkenazim, who generally follow the rulings of the Rama, thus refrain from bathing after immersion. Rav Moshe Feinstein (Russia-New York, 1895-1986), in his Iggerot Moshe (Yore De’a 2:96), held that women should not bathe for an entire day following immersion, whereas Rav Shemuel Wosner (contemporary scholar in Bnei Brak) maintained that a woman may bathe once she even touches her husband after immersion (Shevet Halevi, 5:125).
However, there were many Rishonim who disputed the Or Zarua’s ruling. The Re’avaya (Rabbi Eliezer Ben Yoel Halevi, Germany, 12th century) distinguished between the cases of Teruma and Nida, and claimed that the enactment mentioned in the Gemara applies only to those who immerse to eat Teruma, which is something sacred. It should not, he argues, be extended to the case of a woman immersing in order to become permissible to her husband. And the Radbaz (Rabbi David Ben Zimra, Egypt, 1480-1574) noted that the custom in Egypt was for women to bathe after immersing in the Mikveh. This was the position of other Rishonim, as well, including the Shaareh Dura, Agor, and Semag.
The Shulhan Aruch makes no mention of this prohibition, thus indicating that he did not forbid bathing after immersion. Accordingly, Hacham Ovadia Yosef rules in his Taharat Habayit that a woman may bathe or shower immediately after emerging from the Mikveh. He emphasizes that this applies during both the summer and winter. Some had claimed that a woman should be permitted to bathe after immersion only during the winter, when the Mikveh might be cold and she needed a hot bath to warm herself. Hacham Ovadia stresses that there is no concern at any time of year, and a woman may bathe or shower immediately after immersing in the Mikveh.
Summary: According to the custom of the Ashkenazim, a woman should not bathe or shower after immersing in the Mikveh. Sepharadim, however, do not follow this custom, and thus Sephardic women may bathe or shower immediately after immersion without any concern.