I bought Angry Birds a few months ago (it was on sale for 1.99). I just came back to my PS3 after a 3 week vacation and now it says "Buy Now" on the store item for Angry Birds. So I checked my DL List, and Angry Birds is still there. I go play Angry Birds, and it works fine. But it doesn't have the little red bag there on the store which it should have. This only happened recently, as it was fine before I went on vacation. What's going on? Should I call Sony?
You're fine! The title was pulled about 2 months ago for some unknown reason (probably a dispute with the publisher) and was recently relisted (a couple weeks ago). When Sony relisted it, the PSN treated it as a new game listing.
As long as the game is in your download list, then you don't have anything to worry about.
To put it simple, the game in your download list and what's listed on the PSN Store are two seperate copies of the same game. Think of it as a re-release, like a Greatest Hits edition.