Computer already associated with this computer.....

Hi there,

I have had an iTunes match account for about a year running on my iMac. I am the only person who uses my Mac. 2 Days ago i switched off Match and switched it on again (I was having problems getting artwork to download onto my iPhone) when I tried to switch it back on i got the message saying "This computer is already associated with an Apple ID - You can use iTunes Match on this computer with just one Apple ID every 90 days. You cannot associate this computer with a different Apple ID for 23 days.

I don't understand when I was logged in only moments before?

Any suggestions?

Many Thanks