Losing my best friend

There's this girl that I met at a party about a year ago. I've gone to school with this girl throughout middle and high school, but we never talked until after we graduated. Doesn't really matter, just thought I'd say it.
Me and her really hit it off. I got her a job at where I was working at the time, and over the 8 months we worked together, we built a very strong bond.
I eventually got fired from the job about 3 months ago. At first I thought it would kind of be the end of our friendship, but it turned out really well. We started hanging out much more and got even closer with each other. I had always had feelings for her and she knows that, and I was pretty sure she felt the same way. There had always been a very strong sexual tension between us, about 2 weeks ago we gave in.
We had great sex, like the best I've ever had. She was great, and it probably helped that I have very real and deep feelings for her.
But ever since that day, we've been kind of distant with each other. We used to talk for hours everyday, is not in person than on the phone. Texting all day, you know the deal. But in the past week everything has slowed down. She doesn't call as much, doesn't answer texts. We still talk everyday but it seems very brief and rushed, almost like a chore.
Keep in mind this entire time she has a boyfriend (they started dating right when I met her)
It could be she's guilty for cheating, but we've messed around before just never went all the way until the other day. I really don't think she's feeling remorse.
It's like we've lost our "spark" or something. Conversations seem mandatory, just talking because we're used to it.
I don't know what to do. I really want to be with her, but I want to be her friend if nothing else.
What do I do!?!



Cliffnotes
Fell in love with best friend
We had sex
Things are now weird