Weird message - invalid username/pword

Most times when I visit this site, I am already logged in. One day last week, for some reason it said I was not logged in, so I entered my username and password, and got this message:

"You have entered an invalid username or password. Please press the back button, enter the correct details and try again. Don't forget that the password is case sensitive. Forgotten your password? Click here!

You have used 1 out of 5 login attempts. After all 5 have been used, you will be unable to login for 15 minutes."

However, at the top right corner of the page it said "Welcome, nybiker. You last visited:", etc. So I clicked View New Posts like I always do and everything worked fine.

The next couple of times I visited the site that day, the same thing happened, after that it was back to normal.

Today it is happening again - it says I'm not logged in, then says I used the wrong username/password even though it is logging me after I enter them. Is anyone else having this problem? Thanks.