OK, this is probably something I should have read in the instruction manual but I'm at work and I want an answer NOW!!!

Why would my iPod suddenly start selectively preventing me from playing any of the songs I've purchased via iTunes? The song name comes up on the screen and it seems to start playing but before it actually does play it just goes back to the previous screen/plays the next song in the playlist. It's only happening when I try to play songs I've purchased - everything else seems to be fine. This is driving me mental - any ideas why it's doing this or does it just hate me?