Showing House After Contract

You don't own the house yet, or control it, and the seller has a right to do whatever he or she wants with it.  The seller's agent has a duty to act in the seller's best interests.  But your contract stands; no one could come in and upset it.  They may simply be looking for a backup offer in case someting goes wrong. 

If you're worried about further damage, take some pictures now.  You have the right to receive the property in the state is was when you viewed it before making your offer.