Chapter 84:10 Laws of Carrying: Clothing & Jewelry
10. A woman should not go out [into the public domain on Shabbos] with a
menstrual pad ("moch") she uses [solely] to protect her clothing from
becoming soiled; similarly, she may not go out with an apron ("sinar")
which she wears for the same purpose, unless it is considered a regular
garment (1). However, if she uses the pad or the apron to save herself from
the discomfort caused by blood drying on her skin, she is permitted to go
out with them (2).
(1) Any item which is not a regular garment, and which is worn only to
protect one's clothing from becoming soiled, is considered a "burden"
("masui," that is, an object being carried, rather than a garment or
jewelry or their accessory). However, an item which is considered a regular
garment may be worn out into the public domain, even if it is only being
used to protect one's clothing from becoming soiled (See Shulchan Aruch
303:15 and 301:13-14)