Perhaps this friend of your coworker is frustrated that her attempts at being friendly were taken as flirtation, which I'm sure is a problem that most women with male friends will face at some point in their lives. That might explain why she's cut off communication, if she's annoyed at this kind of thing happening all the while (which it probably does if she's a naturally flirtatious person, assuming she wasn't actually interested in you).

It does sound like you had just cause to enquire, though, if other people were picking up on her flirting with you, but it makes me wonder why your partner was so eager to have you meet her in the first place. At least, in your defence, you sought to find out if she had feelings for you by stating your un availability so, in terms of your relationship, you did nothing wrong.

I guess in the future, if you suspect someone is flirting with you but you're not sure then the best thing to do would be to wait for them to actually make a move for you to decline rather than trying to make a preemptive move. Best of luck with your partner.