I am also a new member and I am sure someone more qualified will be along to help you. people are great about that here.
I have just a couple of questions is your network a WIRELESS connection?? I could be as simple as your neighbor "borrowing" your wireless internet access.
You should probably change your password and set up your ROUTERS security setting.
If your library is set to "shared" (he/she) can see your library as well.
You haven't seen any "funny charges" in your account have you?? That would be a whole set of new problems.
Hope that helps you a little until someone else comes along