iTunes says computer not authorized? oh, yes it IS.

This is driving me crazy - my computer is authorized for my iTunes account.

When I was syncing my iPad recently, iTunes kept saying that certain songs (55 of them, for which I paid iTunes long ago), could not be synced because the computer is not authorized for iTunes.

I checked via iTunes Store and, indeed, my computer IS authorized.

This keeps happening.

WHAT IS GOING ON? Why don't I have access to music I paid for?

I'm not about to delete music I paid for; and I'm not really up for paying Apple Care to help me on the phone (my Apple Care Protection Plan has expired).

iMac

Lion 10.7.4

iPad, 3rd generation