While writing The Tapestry Shop, I researched medieval clothing. Paintings show us the outwear, but little was written about underwear worn in the Middle Ages. What we know about clothing comes from the few extant pieces that have survived the years, carefully preserved in museums with controlled climate and lighting, but with underwear—being what it was—we have little to go by. The Chartres statues, for instance, represent outer garments, so we can only guess, from representations on pottery and drawings, at what was worn beneath. There are representations of women participating in games that show them wearing something that looks much like a bikini, a small lower piece and a binding wrap at the top.
When full skirts came into use, it's doubtful women would lift layers of cloth and then have to untie something to answer nature's call, although something like men's loincloths may have been worn during certain times of the month.
Women wore undergowns, or chemises, beneath their outer gowns. In the picture, this woman has her outer gown tucked into her belt, perhaps to allow a bit of air to pass through her chemise, but this was the furthest she'd go.
Men, in early Middle Ages, wore loincloths like what is shown. Laborers in the field thought nothing of stripping down to their loincloths in hot weather. At other times, the clothes were colorful and part of everyday outer garb, as the picture suggests, and men at sea had no compunction about stripping naked during daytime chores on the ship, unless there were women aboard.
We know more about the hose they wore, as that garment is visible in statues and paintings. Hose were made of two woven pieces of fabric sewn together, usually of wool. Their wool was a soft weave because of the manner in which it was made, nothing like our wool today which would be a bit itchy, at least to this writer. Later, hose (hosen) worn by armored knights were made of sturdier material and called chausses, an item worn beneath the armor.
In the late Middle Ages and Early Modern period, hose became a significant part of everyday outer garb and were frequently colorful and made of fine fabrics.
There are several good reference books on the subject, but be careful to steer away from costume books used for Hollywood productions. Some are not true to the period, but look better on screen. For anyone who's interested, a good little overall guide, one I have on my reference shelf and which gives a good idea of the construction of medieval clothing, is Medieval Costume in England and France: the 13th, 14th, and 15th Centuries, by Mary G. Houston.