Music gone after last itunes update...

So after the last iTunes update some of my music that i have bought from the iTunes store is gone. Now this have happend more than once. Not just after this update btw. I have tried the method from this link here: http://support.apple.com/kb/HT2519

Does not work. Today i just wanted to test something so i bought a song. And yes it workd but then i connected my iPhone to my computer it synced and the song came om my computer. I saw the song it was on my computer. Then i clickd on my computer again sync. Then it was gone. And now i cant find it or download it. I just dont understand what is happening.

Sorry for my bad grammar.