This can be due to your AWWA not being rinsed properly after use.  Try washing them again with a hygiene rinse to see if that helps. If you don't have access to a hygiene rinse or prefer something more natural you can try soaking them in the sink or a clean tub with one cup of white vinegar and some cold water. Leave them to soak for about 15 to 30 minutes, then clean in the washing machine. You can add a little white vinegar to the wash too, if you think they need it. This will remove build-up and get rid of the odour.  Make sure to fully rinse out your undies immediately following wear (especially if you might not be putting them in the washing machine for a day or two). To ensure that they are completely dry all the way through before wearing again you are able to hang them in the hot water cupboard, however natural sunlight is best for your AWWA.