(From an email I sent to a mailing list, where we were discussing womens underwear. Contains TMI about womens bodily functions; if you don't like reading about that sort of thing, stop now.)
Did women in SCA period wear underpants? The two reasons usually given by SCA people or other living history types for needing underwear of some kind are chafing and menstruation.
The first reason, chafing, was probably less in period because 1) if you never wear pants the skin of your thighs toughens up, and 2) there were less obese women, and obesity is definitely a factor in thigh-chafing (I say this as someone who knows from experience).
The second reason, menstruation, is an interesting one. Some people put forth the argument that later menarche and constant childbirth and breastfeeding means that medieval women rarely menstruated. However, if you look at the actual demographics for the Elizabethan period (which is my period) you find that women married (on average) in their mid twenties, and had something like 6 pregnancies resulting in 4 children (the other two miscarriages or stillborn), or something. So let's say 2 years of non-menstruation per live-born child, and an average of 6 months non-menstruating for miscarriages or still births, and you have 9 years non-menstruating between ages 25 and say 40. That leaves 6 years of menstruation, plus say 9-10 years between menarche and marriage. That's not insigificant.
So assuming they had to do *something* about it (because as pointed out, just bleeding into your clothes gets expensive really quickly) they probably would have worn a pad similar to those used in the earlier parts of the 20th century - long, non-adhesive, attached to a belt. You could even pin them to the waistband of a petticoat if you were in a period that wore garments with waistbands. Having some kind of drawers wouldn't help you much with this sort of pad - I don't know if anyone's tried using menstrual pads with boxer shorts, but it just doesn't work. Modern adhesive not-attached-to-the-belt pads only work because modern briefs fit so closely to the vulva. Pads simply don't stay put if you have anything less close-fitting. The belt arrangement keeps things pulled up close and that's why it was used until modern close-fitting elasticised briefs became common.
Another point about modern tight-fitting briefs is this: if you are wearing tight fitting briefs and you have some "spotting" you will stain your briefs. But if you have spotting when wearing *no* briefs and a skirt, you probably won't stain anything. Your own anatomy keeps the little bit of fluid tucked away, even when sitting down. Trust me on this, I've tried it. You need to have real "flow" to stain. Though I will admit that it might be different for people who are a very different shape than I am.
A final point about tight-fitting undies: if period womens drawers were tight-enough fitting to hold a pad in place, they would have been very constrictive and uncomfortable. Remember we're dealing with woven fabrics not knitted, and even cut on the bias something that clings that closely to the crotch is going to make it hard to sit and bend.
So the remaining part of the puzzle is some pictures we have from period of women wearing pants, plus one or two extant pairs of Italian drawers dated to ca. 1600 (and shown in Cunnington's "History of Underwear"). In the various discussions I've seen on other lists, people have said that women-in-braies iconography often has to do with stories about henpecked husbands... "She's wearing the pants" and so on. I'm not familiar with those pictures but it seems like a reasonable argument.
But even assuming that the rare pictures of women in braies are depicting actual usage, and that the extant Italian drawers are correctly dated *and* were worn by women, that still doesn't mean that all women wore drawers all the time. There are *lots* of pictures of women in various states of undress that *don't* show underwear. So perhaps drawers were only worn for certain special circumstances... horseback riding being one option, for instance. Travel in extremely cold weather is another. (Sitting still in cold weather it doesn't make much difference if you wear drawers or not - it's moving around and letting cold air up your skirts that makes a difference - again this is something I know from experience, as I live in Canada and attend SCA events with no underwear in all weather.) For the reasons mentioned above I don't think that menstruation is a particularly likely reason. But there may be others.
In practical terms, for modern use, I personally choose to use tampons and a sort of anti-chafing stick they sell to runners (it's called Bodyglide and comes in a stick like deodorant, it's colourless and odourless, and you just put it wherever you get chafing, and it lasts a full day at Pennsic.) These are both basically invisible modernities that let me dress in a way that feels "right" to me even if nobody else knows what I have (or don't have) under my skirts.