I think she should be getting a more thorough answer from you or she will go elsewhere to get the answer.
You answer has been for older girls. Well, 3rd graders are older girls mom. Think about it from her perspective.
You don't have to describe the 'bleeding" as something to be scared of.
Quite honestly, right now in 2nd grade, she has heard much more than you think she has. I suggest some open and honest communication which is matter of fact so she understands. It is not threatening to have open lines of communication with no topic off limits, unless it threatens you somehow.
My daughter is 19 and when she asked the question LONG ago, less than age 5, I just answered her appropriately.
If you have an issue with speaking so candidly with your daughter, then go on a drive so you are not looking at her and if that is not doable, invest in some books so she can learn.
It is scary at all the info that the children speak about while at school regarding sex, STD, bleeding, etc. You DO want you daughter to trust you enough to come to you for the answers.
Best wishes mom... they grow up way to fast