this happened to me a while back. Said I had bookoo PMs to answer... But going and checking, I found there weren't any new ones.

What happened to me was that I started rifling through old PMs from before the changeover. Some of them have never been Read on the new VBulletin format here, so this causes the glitch to tell you that you have lots of unread PMs, more than you've ever received even. After a little while the problem just went away on its own.