Really, unless she tells you what she is thinking, you really don't know. If you've asked her what she's thinking and she doesn't want to tell you, then you're kind of out of luck.
From what I understand, you've gone from friends to FWB. She was ok, until you expressed deeper feelings. Not all FWBs want to take the relationship to a deeper level, mono or poly.
I'd say, slow it down. Let things progress at a pace she's comfortable with. Try to get her to open up, but don't press the issue.
You may end up with unrequited love, which sucks. But she's not leaving the relationship altogether, which is a good sign.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR