This is third night I've developed a headache, then stomach cramps that lead to loose stools. I feel fine til evening. A 'bug' should last all day so?

Yes but???? This is only happening in the evenings. Are you at work during the day? Could something in your environment be causing this? Is it something you are consuming in the afternoon or evening hours? Any change in your medication? These are all things to look into and if symptoms persist,you definitely need to see a doctor for evaluation.