Thank you for reaching out to us. We are glad that you thought to ask about this.
Rema (Yoreh Deah 201:75) rules that a woman should not bathe right after immersion in the mikveh. This is based on a concern about giving the mistaken impression that an ordinary bath makes a woman tehorah, and not specifically immersion in a mikveh. Ashkenazi halachic rulings typically follow this approach, which applies to either a bath or a shower.
Many Sefardi halachic authorities permit showering at the mikveh following immersion. Additionally, some Ashkenazi authorities permit showering at the mikveh when necessary. For example, the Tzitz Eliezer (11:64) permits this and recommends that a woman in such a situation first rinse off her body and then her head, which makes clear that the shower is not an act of purification.
In your case, where not showering at the mikveh poses a special stress or health risk on mikveh night, it would be permissible to shower at the mikveh, preferably following Tzitz Eliezer’s advice to wash head and body separately.
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