iTunes authorization problem?

I have a total of 95 songs in my iTunes library that I want to sync to my iPhone. I have bought these songs on 3 different computers, transferring these songs from computer to computer easily. I last synced music to my phone on computer #2. I buy a song on computer #3 and want to sync it to my phone. Before I sync it, I realize that I haven't set it to sync music. I select sync music, then sync, then it asks me if I want to erase and sync, so I decide, "Why not, the songs are the same." While syncing it says that I don't have authorization to sync or play 20 of my songs. I'm a little weirded out, but I authorize the computer. I hit sync again and it still tells me that I'm not authorized! What's the problem?