From time to time, the rather small amount of surviving medieval garments grows a bit bigger by favourable circumstances and a lucky find. And such a thing has happened fairly recently, at Schloß Lengberg (sorry, links will be all in German!) in Austria. When renovation works started in the building in 2008, archaeologists were making a spectacular find in the fillings of one of the spandrels of the vaulted ceiling: Lots and lots of small finds dating from the 12th to the 18th century - playing cards, coins, shoes, wooden, iron and copper implements, glass, paperwork and - be sure you are seated - garments. And these include underwear!
You can read a bit about the textile part of the find here, and if you do not read German, I still recommend to take a look at this .pdf, an excerpt of the magazine "Harpfe" - there are pics of some of the small finds, including a flute, and there's a picture of one of the underpants found in the spandrel.