Advice???

I had a fwb for about a year now (off and on) I lost my virginity to him. And I've never really felt any kind of feelings towards him, not enough to want to be in a relationship at least. He now works with me, and all of my coworkers keep saying "you and him would make a great couple". And he's acting different with me, I can't tell if he's being friendly or if he's flirting. We had an agreement, no relationship. But idk, what if it's changing? What if I want it to? I don't know how to deal with this kind of thing.