If she feels she doesn't need to know all that stuff yet, and is hiding it from you, then she really does need to start learning about it. She's frightened and worried about the unknown. It has entered her head, she does know something, but it doesn't make sense to her. It's up to you, as her parent, to prepare her and start teaching her.
Sex education doesn't need to be 'taught' formally. It can be the result of a cosy discussion. But if it scares her so much you need to address it.
I am determined that I will teach my children about sex. I don't want their first knowledge to come from school, to seem academic and soemthing outside the family. So, as ds1 (my PFB, and also a sensitive, easily worried type) hadn't asked any questions by the summer holiday before he entered Y4, I engineered the discussion. It came about because he was in the room while I was dressing, and commented on my fat tummy - how did I know that there wasn't a baby growing inside when it was so big?
Perfect jumping-off point!
Short question and anwer discussion resulted in him being delighted "You mean that when I'm an adult I'll be able to get married and make babies with my wife?! Oh Wow! That's brilliant!"
I was quite suprised at his delight, and at the happiness the discussion would give him.