A best friend competes w/you for your new friend?

This year started out tough. I'm SO shy but I was placed in different cles than ALL of my friends, including my closest: Val. My other friends share several cles together. I was able to make friends with another girl, Lu, who's in most of my cles. Lu's been at my school many years but this is the first time we've ever had cles together. Now, out of the blue, Val is aggressively pursuing my new friend Lu. Val has never been interested in a friendship with Lu until *I* became friends with her. Coincidence? Val has a ton of other friends but Val runs up to Lu like they're long lost best friends & Val barely notices me. Val announced to me that she invited Lu to her house. Val tries to sit next to Lu on the bus as often as possible so I can't. I feel like Val is trying to compete w/ me for Lu's friendship & Val can't stand to see me have a friend NOT connected to her. I know I sound over-possessive but Val's manipulations are making me crazy! What can I do?