Most Helpful Girl
It was all fun and games for her to date someone new. Then, when she see's that some other girl is putting value in YOU, it makes her wonder how important she actually was to you. She wanted to the one you couldn't replace, but you did. That bothers her, so it was really all about her ego, wanting to beat out the other girl, it wasn't about her feelings for you. Wise move not falling for her shit.
Most Helpful Guy
Just stay with the one you have and ignore the other one. She had her chance and wrecked it. She wants you more now because you have moved on.