As far as telling whether or not it's urine, you could maybe check out the smell -- I think even a few drops of urine would be noticeable. That said, I would be surprised if it were incontinence, as having vaginal discharge stain or discolor underwear is not an uncommon issue at all. :)
It's been happening to me since puberty, too, and I'm pretty confident that it's not due to incontinence but just to the action of my normal vaginal discharge. Mine will discolor white underwear in pretty much the same manner you describe, and sometimes will even bleach darker-colored ones. This is also something I know other people have experienced -- this entry (http://community.livejournal.com/vaginapagina/12272307.html) is about the very same issue as experienced by another VPer.
So I don't think there's anything wrong with you or how you wear underwear. You may be different from your mother (and your boyfriend's mom and sister) but that's not a bad thing. :)
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It's been happening to me since puberty, too, and I'm pretty confident that it's not due to incontinence but just to the action of my normal vaginal discharge. Mine will discolor white underwear in pretty much the same manner you describe, and sometimes will even bleach darker-colored ones. This is also something I know other people have experienced -- this entry (http://community.livejournal.com/vaginapagina/12272307.html) is about the very same issue as experienced by another VPer.
So I don't think there's anything wrong with you or how you wear underwear. You may be different from your mother (and your boyfriend's mom and sister) but that's not a bad thing. :)