Ah, oh crap, I suppose that could be it. I'm getting the 'You don't have permission' page (sorry, should have offered that detail to begin with).
I've been using a subconscious combination of 'I've still got my custom tag' along with 'I haven't received any kind of notification that it's lapsed'. I guess either or both of those could be wrong.
That, I can't tell you -- I don't know that there is anything in one's profile to say when it lapses. Hopefully a mod/admin can stop by and check for you. I think there's an e-mail notification that goes out when it expires but as I'm somewhat of a special case I haven't gotten one myself so I can't say for sure.
I ran into the same problem. If you signed up recently, you get an e-mail. However, I signed up a little over a year ago and had no idea it was about to expire. I got lucky and saw something that reminded me it expired that day.
To find out when/if your account expired, click on Support the Site! at the top and then click on Community Supporter Accounts.