Usually this happens when you have done something that made her reconsider (in this case often there is no boyfriend) or she did not have a drink for more than half an hour and begins to have second thoughts about cheating on her (existing) boyfriend.
You: "Did I tell you that i travelled the world for the last couple of years?"
Her: "Aaah, that sounds interesting. My boyfriend went on a three weeks vacation last year too."
You: "Hey, dont boyfriend me! (off you go)"