It's typical for women to start seeing the gyno at 18, it's actually advised by most doctors because you can no longer go to the peditrician for overall care and the new adult doc you get will proabily not have time for you. Tell your mom you're worried about your health, and that's the place the doctors want you to go.
Also, from your profile it looks like you're approaching 18 anyhow, at that legal age (at least in the states) you have the rights to your own body.
I think you definately need to be examined by the gyno, along with the STD tests run. If you think you're responsible enough to have sex you need protection, and your mom should be HAPPY that you're seeking it out!
