Could there be something dead and decaying inside the floor, like a mouse? Also, those drains should have one way vents on them to keep the sewer odors from backing into the house. Run a mixture of ice and warm water through your garbage disposal - I've found that they can stink up a whole house if they haven't drained completely.
A good place to start, that may not cost you anything is to call the inspector you used before you bought the property. They would be highly motivated to help you find out what is going on since they would have been the ones you paid to identify these kind of problems before your purchase. I absolutely agree about a call to the realtor.