What avast version are you using 4.8 or 5.0 ?
Avast 4.8 support is likely to cease at the end of this year so there are notifications to 'update to 5.0' that however isn't a re-register notice.
Avast 5.0 doesn't remind you to re-register until 14 days before it is due to laps. So if you have avast 5.0 then I would suggest that you check your system date and adjust as required.
An image of the pop-up would be helpful.
