If this has only just started happening it could be the hot weather - you have a very short time between washing cycle finishing and a nice pongy smell developing when it is this warm. If you are n the habit of putting a wash on and going out, then that might be it. You have to get the washing out of the machine pronto when the cycle has finished in hot weather.
If the drum is pongy either run a cycle with nothing in but some laundry liquid or powder, or even a little household bleach, just to scour out the gubbins.