I think I saw these types of pants on TV when I visited Tokyo a couple of months back (Or atleast something that looks alot like these pants). They were these exercise type pants that people wore under normal clothing as they went about their every day lives and they claim it helps them lose weight.
sgcdonmai said: it looks like she's getting off on the sensation of the material on her skin.
Judging by the artist's other illustrations like post #123269, I'd wager that unintentional stimulation is the case rather than the scatological scenario. Many women are able to masturbate without the use of their hands (or other external object) if they're wearing sufficiently tight clothing.