As others have said, it's not too hard for someone in his position to see your requests going out to LJ. Each one carries your ID and password (although the password is mildly encrypted). He could send requests for pages that appear to come from you. This would be rather tedious to do if he is just browsing to see what he can find. If he can figure the decryption of the password, he can log in as you the normal way.

Is he trying to find out private things about you?