I've had the same problem, and honestly, I've come to just accept that something in my vag - I'm guessing something to do with the acidity or PH - changes the color of my underoos. Black underwear gets a kind of bleached effect, and white underwear gets a yellowish staining effect. If you're concerned, you can certainly go to the OB-GYN and have them test for abnormalities, but honestly, I'd just say that this is how your vagina works. If you aren't having any problems - if you have an otherwise healthy vagina - leave well enough alone, and tell your mom to stay outta your crotch's business.

Everyone's body works a little different. As you can see from the responses here so far, you are NOT alone. I very, very, VERY much doubt this is incontinence, or you not being "clean" or something like that. Vaginas are interesting environments with all kinds of biology & chemistry type stuff going on. So try not to worry about it. You're not alone. :)