What are my rights when the real estate agent has a showing and wants to come in my side?

As a tenant without a written lease, assuming you pay your mortgage on a month to month basis, you are essentially a month-to-month tenant. Regardless of that, the landlord should have the right to show the property (purpotedly to sell it?) upon reasonable (24 hours) notice. Most leases would provide for this, and while you do not have a lease in writing, a court would most likely hold that this is reasonable to enable the landlord to either sell the property or to find new tenants to rent to.