I had a problem a couple of weeks ago:
http://www.golden-road.net/index.php?topic=22113.0, and I believe I have figured out why that happened, and it involves what you are experiencing...
I always login to the site from my computer, iPad, or phone. Lately, I've been accessing the site from computers on campus in the library, and for some reason, whenever I go to log back in on my normal computer or wherever, I have to re-enter my username and password. Because this information is already stored on my devices, it's automatically inserted into the username and password fields, but when I click login, it says my password is incorrect. I then have to erase the password and re-type it in. It's a little annoying, but I've at least figured out the root of the problem.
As for you, have you been logging in from a different source than usual, or even a different browser? If so, then that's your problem. As far as fixing it goes, I have no clue. Maybe Seth knows something we don't.