I actually had something similar happened with my daughter around the same age. I wrote my daughter a brief note told her that I had found it, that I did not want her at all to feel embarrassed but I wanted her to feel like she could come to me with questions. I also left her with a notebook and told her when she had questions to write it down and leave it under my pillow at night. I would then answer her in the same book and return it to under her pillow. For my child it was much easier to write things out than it was to sit across from me at a table and talk. She was able to express herself feeely this way. It also gave me a chance from time to time to compose myself before I replied so that I didn’t appear shocked or worried or anything that might make her uncomfortable to the point that she no longer felt she could share with me.
I have always felt is very important that we have these discussions with our children even when it’s uncomfortable for us – maybe especially when it is uncomfortable for us. Since writing was easier for her it was the perfect solution. It might not work if there are siblings in the house who could intercept the journal, though, so take care with that.
In the end it really comes down to giving her a chance to communicate with you as freely as possible. As uncomfortable as it may be for you this is definitely a discussion you want her to have with YOU and not with others. This way you can be sure she receives accurate information that conforms to the values around this discovery that you want for her to have.

Without getting too mad, ask her what they are and how to use them. Make her think she knows something Mom doesn’t. And have her demonstrate the proper way to use then, this way you can assure yourself she isn’t using them improperly to hurt herself. You may just enjoy the lesson…