Keep it going bro, and if its still a secret you can keep it that way, let her think that shes more sexy to you now
I did that with one girl I did actually love years ago, and she was special.
Anyway, my penis was showing growth to her, like with your lady, and comments were made. So, I told her that she was turning me on so much, and was so damn incredible that my cock only got that large for her.
She believed it, and we had some of the most amazing sex I have ever had. Tell her this, if anything else, the girl will be very flattered. Its not being dishonest, I assume she does mean something to you.
All the best