Hello, I had an internal ultrasound (to get a better look at an ovarian cyst) when I was 18 and still a virgin. I freaked right out when I saw how long the probe thingy was, but they don't really put very much of it in. Really, it's not that bad. You might like to speak to the Sonographer about it when you first see them, but I'm sure they will be very gentle and it will be ok.
:-)