If you look in your iTunes account (View My Account from the Account or Store menu), if that only shows that you have 2 computers authorized, yet you can't add another iTunes device (computer), then I would suggest that you click the "Deauthorize All" button, then re-add your devices.
If that account shows that you have 5 computers authorized, then you would still click the same button, again re-adding each computer. It would be a good opportunity to make sure that the only devices that are authorized, are those under your control.
You re-add from iTunes on each computer that you want to have as authorized to play content purchased with your AppleID.