When can this occur in a bitch and is it only in a bitch that has had a litter of pups

It can happen at anytime & occurs in maiden bitches & bitches that have had litters. There are two types of Pyometra closed & open, the closed one is more dangerous as there can be no signs until it is too late. Most Pyometras occur after a season, but I've had bitches that have had them at other times. If a bitch is never going to have a litter I would always advise spaying when she is mature.

My friends lab was spayed when her pups were 2 days old due to pyometra.