My sister has had some interesting times with her 2 DD - who are now 12 and 15, but one thing she keeps stressing to me about everything they discuss is that she keeps the lines of communication open. I guess you can be there when she needs to chat, and if she asks very specific questions about sex etc. you could turn the conversation back to her, by saying, well what do you already know? why do you want to know specifically about...? and that may make it a bit easier for you to answer her, and understand where the questions are coming from. I do this with kids at school all the time - by turning the questions around back to get them talking you find out a bit more info and then that may mean you dont need to give as much detail as you think iykwim.

It may also be a phase that is being fueled by other kids at school. It may be worth having a really casual chat to her teacher (maybe without her knowing ) to see if they HAve noticed anything with groups of students or they may have talked about something in class that may have triggered it! I know at my school we crack down pretty hard on the grade 5/6 students when the whole boyfriend/girlfriend issues come up, so they might be able to shed some light on it.
HtH! Good luck hun!