Smelly bottoms can be a sign of a parasite infestations. Swelchy smelly damp bottoms are a dead giveaway this might be your problem. You may want to look at the parastie boards for some ideas on doing a parasite cleanse.
Another reason for fecal smells are fecal loading, meaning constipation. Do you have regular bowel movements? If not, this also could be your problem and you might want to look into some bowel cleansing routines.
At any rate, a parasite cleanse is a good idea to start.