Unless the carpet took an age to dry, the cleaning probably did re-activate some odour causing nasty already in the carpet.
Often they are not noticeable once thoroughly dry, but if the smell persists, without going to the extent of lifting the carpet to see whether it is coming from the underlay (which if it is, would need that section replacing) you could heavily MIST an effective deodoriser over the pile and brush in, then allow to dry.
Make sure it is a DEODORISER and not a just a perfume or scent. A proper deodoriser will break down the odour causing bacteria, a perfume or scent will simply mask the smell for a short time, only for it to return.
Dave.