Turtles like many reptiles and birds produce eggs - regardless if there is a mate there to fertilize them (chickens

) It just sounds that your yellow belly is producing eggs because she is a female and its in her nature, not because there is a male in the tank.
You will want to provide a cool place that is out of the direct light (so slightly shaded) with a medium for her to make a nest out of - I have seen soil, sand or reptile moss all used for this.
When she is ready she will lay her clutch and move on...