Sorry, but I have a boyfriend

Don't ever, never, ever ask if she has a boyfriend! Maybe she does, maybe not - so what, who cares?

Maybe she does, but wants to have a little fun - your prompting about a possible borefriend will put her off the mood, she'll move on to another guy and get what she came looking for anyway. Or maybe she does, but is also pretty curious about you and wouldn't want the existence of a boyfriend scare you away. She might even be considering dumping him in favour of you. Once you've extracted that information from her however, she might just write you off in her mind.

If however she blurts it out without prompting, it still doesn't mean anything. Maybe she really thought she wanted to scare you away, but she really didn't know what she was doing Take it humorously, be light-hearted and give her another chance