Here's the thing man: the least attractive thing you can do to/for a lady is to be jealous/possessive.
If you don't act jealous and she's interested in you, she'll hang out with you and you'll both fall madly in lust. If she's interested you and you act jealous, it'll be a definite "no."
Don't shoot yourself in the dick. Give her the room she wants. What's the worst that can happen? She likes this other guy? You don't want to be with someone who would select someone else over you (trust me on that one). Best that can happen? She'll go hang out with that dude and realize that what she wanted was to be with you in the first place.
It's really difficult for people to make the decisions without enough information. If she's going to chose you, she might need to know that other people aren't as fun/exciting/(another of your traits here) as you.
yea i know i can't act jealous or possessive but damn it's tough when ya get blindsided like this