If you weren't going for her then you should have called her out on her behaviour "That's very nice, I saw the ring earlier." I'm a little confused though, in what manner was she doing this? Was it a "don't try anything with me" sort of thing or a jumped in your way and stopped you conversing with certain people thing?

If it's the getting in the way thing then who knows, some sort of event between the earlier meeting and now? The group could have had a chat about who they thought you 'fancied' and that person decided they weren't interested. Or they could have met some other guys during the night. Or any number of other reasons to be honest.

Sometimes it's good to plow just to work out exactly what the problem is.