If the mop head is dirty and smelly then this could be the reason for the foul odor. If the mop head is not dried properly from the last use, then this can also lead to foul odor. If you are experiencing this, soak the mop in hot water, add bleach and soap into it. Instead of bleach, you can use vinegar as well. This way you can get rid of the smell. Even if this process is inefficient then it is time to replace the mop head.