Our bedroom is in the attic and has an insert chimney and lately when we flush the toilet down stairs the whole house smells but the worst odor is in the attic? What could this be from?

Hey minand welcome to The Plumbing Page on AMHD.
You piggybacked on a 6 year old dead thread so I gave you one of your own. Before you post look in the upper left hand corner for the date.. I can only give you a situation where this could occur. If there were an partial clog in the toilet drain line setting up back pressure and there was a open vent in the attic, the sewer gas could be blown back up the vent.
Have you considered a smoke test? Let us know what you come up with. Good luck, Tom