Do you have a rental agreement? A written agreement should state who is allowed to live on the premises and it should include a clause regarding guests and how long they are allowed to stay. For example, my lease agreements list every tenant - by name - that is authorized to live on the premises permanently. It also spells out that guests are allowed to stay up to 14 days. A girlfriend staying over on occasion, even for the weekend, would be legally construed as a guest, not a tenant.
Unless the Landlord put something in writing, you are within your legal rights to have a friend, girlfriend, or other guests stay with you on a temporary basis.