iPod touch Itunes - Problems buying songs

If I try and purchase a song on my touch I get the message

"Your account information has changed. Go to the iTunes store on your computer and update your billing info."

But I can buy songs fine from my computer fine and no information has changed?

Any ideas??

Have tried this my ipod is all fully synced has been for a few weeks, I have also tried purchasing songs while connected to the computer?