Sounds like a dead skunk crawled in and died or some other animal or rodents. clean it out and spray an odorcide type odor killer all around and see before new plastic although the odor may be coming from under the plastic so that might be a start. This has been a wet year in places and maybe it just needs a chance to dry out before putting down new plastic. Also try incorporating some lime into the dirt. Bags of lime are cheap. Maybe add some bentonite for any soil moisture.