Your Fiddle-leafed Fig is reacting to your not watering it enough. This is a plant that doesn't tolerate dryness well at all. It should be watered thoroughly so that all of the soil is completely saturated as soon as the surface of the soil feels dry. I would expect that would be about once per week. If it takes much longer than that, there is some other issue.
Make sure your tree is right in front of one of the windows and not tucked in the corner. The more light, the better. Of course, make sure the window is completely uncovered during the day.
Like most plants, when it is under stress (in this case from excessive dryness), the plant reacts by sacrificing some older leaves while pushing out new leaves. So it will continue to lose and gain leaves simultaneously until you provide more water. Then the number of older leaves dying should be reduced while healthy new growth continues.
