yeah, she's kinda infested with those littel dudes... too bad you only find out in the game, because the anime only covers 1/3 (Saber route) of the whole story, leaving Rin and Sakura's rout kinda out.
In that, it's the cause of her being taken over by the darkness in the *game*, and the Sakura/Rin fight in the anime is a (much inferior) rip-off of the end of the Heaven's Feel route of the game.