When I use my shower, I have a very small amount of water leaking to the basement (a small puddle on floor). It NEVER happens when the tub is used for a bath. It has a fiberglass surround that was installed about 6 years ago, but the leak just started about 4 months ago, and doesn't seem to happen with every shower.
Thank you for any help.

Did someone new move in about 4 months ago or did you change something in the shower. Usually this kind of irregular leak is traced back to a particular bather. One who gets more water on the walls then anyone else. Usually this type of leak is coming from an inproper seal around the handles or trim of the valve and sometimes it is because of a cracked shower arm.