Why do you have to reset your password? Because you can't log in or because you're asked to change your password because it has expired? The later is not enabled on normal accounts. So we'll assume it's the former.
If you can't log in and you get an error about invalid username or password, you absolutely are not entering in the correct username and/or password. The system does not make that type of error looking up your username and password and calculating the correct value just as a calculator does not error on a simple 2+2 equation.
Note that passwords are CaSe sensitive while usernames aren't. The only other possibility is someone may have access to your account and is changing it.