I had that happen several times at the end of last month (and once or twice since). As I understand, it's because Dagorlad & the guys decided to step up the site's security measures after the hack, and because we've recently had a spate of spam posts.
Plus, I've recently changed my online accounts to a new set of passwords, and every time I do that it takes a while for me to get used to putting the new ones in right.
Haven't had much trouble with it since, though.
I wouldn't worry about it too much. Just double-check on what the site says you've posted recently, in case it's because someone managed to sneak into your account at some point.
