Bad aftertaste. Idea what this could be?

This could be due to stomatitis or gingivitis which is caused by plaque buildup in the mouth whether on the teeth, over the gums or the tongue. Proper clinical evaluation will be required to assess the present situation so that the right treatment options can be explained it to you. But most likely you will be requiring a session of teeth cleaning by a procedure called as ultrasonic scaling. Post operative maintenance by a meticulous tooth brushing technique (as advised by the dentist) will help in maintaining the results in the long term. Cleaning your tongue everyday is equally important and it should be done only with your toothbrush and not with any sort of metallic or plastic or wooden tongue cleaners. Maintaining a good level of hydration also contributes to improve oral health. Regards