Hi. Thanks for your query. Noted the history and understood the concerns. Stomach flu causing dehydration is not a good sign. You should get intravenous fluids and electrolytes to avoid dehydration and electrolyte imbalance. The cause of the swelling of the half of upper lip can be one of the following: Angio-neurotic edema. Allergic reaction to something. I would hence advise you the following in such a situation: Get a clinical evaluation done by your Doctor to get a probable diagnosis and hence investigations of the blood, urine and stool to get a proper diagnosis so that a proper treatment of the causative agents be done and symptomatic treatment carried out for the swelling of the upper lip. If there is an infection, you need a course of an antibiotic and supportive medications till cures. You may need an antihistamine and steroids for the swelling of upper lip.