Try the "forgot password" option so you can make sure that the password is what you think it is. With all the various accounts everyone has from online bank, credit card, various online sites from bookstores to movie retailers and rentals to e-mail, let alone work passwords (which usually have to change every 30-90 days), you may have confused them -- no matter how sure you think you are.
As an example - I tried logging into my DDD account a few days ago. I got an error saying that my userid/password was invalid. I KNEW I had NOT changed it; I had the correct userid. So I used the "forgot password" option and got my password. Well, as it turns out, for whatever strange reason, DDD had truncated my password to 6 characters -- yup, the first 6 of what I originally had. Well, I tried the one they sent me and ... Presto, it worked. I then immediately put the password back to the way it was before truncation. Tried to login with the new password, I just reinstated, and all is well.
my 2 pennies.