Here Today... Gone To Hell!

So I just began dating this girl that I work with. Everything is great. Since dating at work is kinda frowned upon, not illegal but you know they'd prefer it didn't happen we agreed to keep it on the downlow for the time being. Out of the 120 people we work with only 12 know, and only 5 of them are completely aware, the other 7 just assume.

So I told one of my friends who doesn't work with us about it and she told another of her friends (who does work with us), who just happens to be my best friend about it, yes I was gonna tell her about it, i just felt that it wouldn't be just to do it in any other way but in person. so she came in today to see me today while i was on my lunch just chillen with my new girlfriend in her department of the store. So being that she happens to be my best friend i end up excusing myself so i can go clock back in from lunch and chat with her about the fact i have a new girlfriend.

So a little later I find my way back to my girlfriends department, and she's mad that I told another co-worker about the fact we are going out. I try and defend myself saying, she's my bestfriend, I've known her for years, she needs to know. Then my girlfriend comes out and is like you can so tell she has a crush on you, i could see it just by the way she was looking at you right now.

So my problem is this, how can I explain that, that path has been traveled and that if she really did have a crush on me i'd probably have been dating her for years by now. I know for a fact she doesn't have a crush on me, but how can I explain it without saying how I know that.

And how do i explain the fact that being that she is my best friend that I still plan to talk to her as much as I currently do and hang out with her, no matter what my relationship status is

just my two cents, but you just have to tell her...usually though, I would think that one's partner (boyfriend or girlfriend) assumes that "the relationship" sits in the front seat, what you're basically saying is that regardless of who you're with, nothing is going to change just because you're dating...she gets no perks, no special time, no nothing...that's fine...but don't expect her to "dig on that idea too much"--also, for what it's worth my former manager had a saying with regard to dating people in the  office:

"don't shit where you live"

thanks