Aha ! It sounds like chafing to me, of the sort I used to get when I was a wee boy in Scotland, shuffling to primary school in the depths of winter, wearing my knee-length charcoal-grey flannel shorts and wellies ! 

You’ve possibly discovered what generations of Highland soldiers endured for years. All you re-enactors out there ! The Highland regiments used to wear “hard” tartan, that is, government issue kilts made from a hard-wearing cloth which stood up to the wear and tear of campaign life better than other woven wool tartan cloths. In photos of the 1840’s to 1860’s, you can recognize it by the fact that you can’t see the sett too well and the cloth looks shiny. Officers’ kilts used softer wool and photos show up their setts better. However, Queen Victoria in 1871 (I think that was the year) noticed that her guard of honour at Balmoral, from the 93rd Sutherland Highlanders, had knees which were chafed and red and in many cases, cut and bleeding. The problem was made worse when kilts were wet (eg after fording a river). So she decreed that in future all her beloved kilted Highland soldiers would wear kilts of softer tartan – and so it has been ever since.