How to cure malnutrition?

Yesterday, I woke up, didn't eat breakfast because I wasn't hungry but I normally eat breakfast, and then I went to a cross country race. I ran the race, and was under shock for a while. I had a Gatorade, and a powerbar, and was ok. I went home and from 1 o'clock pm till I went to bed at 3am, I have had malnutrition symptoms. I havent been hungry, but I think I need to eat. Allergy season is in session, and when the allergies hit my sinuses, I have bad drainage, and the mucus gives me a stomach ache, and a lack of appetite. Should I eat or not? All I had yesterday was a power bar, and half a piece of bread. Please help!