Including--
Yeast infections, PID, cervicitis, dyspareunia, endometriosis, insterstitial cystitis, urethritis, vaginitis, vaginismus, and vulvodynia.
The most common reason being vaginal dryness.
This is why we're saying she needs to go to the doctor. Some of these conditions can cause permanent damage or infertility. Yes, it is probably just her being nervous or not ready. But it might not be, and that is why she needs to go to the doc.
If you're mature enough to have sex, you have to be mature enough to go to the doc.
Maybe she's not really ready for sex? Why don't you wait for her to initiate?