I would wonder how much truth she is telling you to save her own ass. It doesn't sound like you are going to get an honest answer from her if she's afraid you'll boot her out. She might very well be in love. She seems to be behaving like that. A fun fuck session with a friend doesn't usually warrant days of constant texting to the point of forgetting where your car is. At least in my opinion.
She could be in love with both of you! This site is about and for people who are loving more than one, not the open relationship description you seem to be putting her into. That of fwb's.
This has only been a couple of days for you. I think you need to take a breath and let things unfold before deciding everything is one way or another.
Anyone want to be friends on Facebook?
Send me your name via PM