'you have downloaded 1.5 Gigabytes today' and i haven't been near the site for a week!
If you have a dynamic ip, that means that it changes every time that you connect (which you already know). Since IP addresses are recycled, there is a very good likelihood that you were assigned the IP of someone that may have just downloaded something. That would give you the message "you have downloaded 1.5 Gigabytes today". If you really are getting an error message that says 'your IP address is already downloading', then someone is piggy-backing on your signal, and coincidentally happens to start downloading at the same time you are. Unless you are part of a network, or you have wireless, that isn't possible.