Here, I stumbled across a post I typed 15 years ago (in 1998) where someone else asked exactly the same question as you did:

https://groups.google.com/forum/#!ms...I/ODstdQD8O5UJ

It gives a fuller description of what's happening, but the bottom line is that it's bacteria feeding and multiplying in your carpet that's causing the smell.

The smell will subside as the carpet dries out, but the bottom line is that your old carpet needs to be replace with a new one or with another kind of flooring that doesn't allow bacteria food to accumulate in it.

PS:
In that same Google Groups thread, there's some guy that responded to the same question that claims to be a Master Carpet Cleaning Whatever and keeps citing lack of sufficient training as the cause of problems like this. If this guy knew what the problem was, he'd spit it out, but he fails to even mention the word "bacteria".

Take a look at this link I cited at the end of that thread which says:

Most offensive malodors encountered in carpet cleaning are the result of bacteria feeding on a food source. Some of the bacteria that remain in the carpet after general cleaning continue to be killed by the deodorizer active long after the professional cleaner has left the premises.

If lack of training of the person cleaning the carpet caused problems like the carpet smelling afterward, then homeowner who rents a Rug Doctor at a supermarket would end up with this same problem. Most of the time it doesn't happen because most of the time the carpets are newer and don't have as much accumulated food in them.

But, the bottom line is that old carpet of yours is at the end of it's useful life and needs to be replaced.