23 female, sick for 3 weeks diarrhea after almost every meal, constant nausea, worse after eating, loss of appetite, fatigue, severe discomfort in the stomach?

This was not a trivial viral intestinal tract infection which lasts a week. You'll have to have a stool culture to check for bacterial pathogens, parasites & ova( eggs). If negative you could have irritable bowel syndrome or inflammatory bowel disease. Consult a gastroenterologist. In the meantime follow a soft,bland diet and,if possible, obtain a prescription for an antispasmodic like dicyclomine