Hello dear... Nyquil is a preparation containing a pain killer, a cough suppressant & an antihistamine...which is generally used to provide symptomatic relief from common cold. However the gastrointestinal symptoms as mentioned in your post such as diarrhea with a lot of mucus,constipation, heartburn ,random pains and aches suggests that you might be having Irritable Bowel Syndrome. Management includes: 1. Dietary modification- Avoid foods and drinks that stimulate the intestines (such as caffeine, tea, etc.) 2. Antidiarrheals such as loperamide 3. Antispasmodics for symptomatic relief 4. Low doses of tricyclic antidepressants to help relieve intestinal pain 5. Rifaximin, an antibiotic So...it will be better to consult a Doctor for further management. Wishing you a Good Health. Let me know if I need to clarify further. Thanks & take care.