Perhaps you never installed a trap under this shower, and sewer gases are moving up the drain. If you did install a trap, but use the shower irregularly, the water in the trap may evaporate and let through the gases. Pour a pitcher of water into the drain. If the odor goes away after a day or two, the problem is evaporation. Run water into this drain regularly. The problem may also lie in the vent system. Have someone open a few faucets upstairs and flush a toilet or two, while you carefully listen to the shower drain in the basement. If you hear a gurgling sound and can still smell odors, then the vent pipe is not working properly. This pipe prevents water flow from sucking water out of the shower trap. Make sure that there is an air vent on the drain line immediately after the trap. This is either a PVC pipe that goes up and into the vent stack, or a simple "vent valve" that is very useful when a vent is difficult to install.