Hello, I really appreciate your concern. Gastric irritation and diarrhea are common with use of antibiotics and pain killer medicines after appendix surgery. For stomach pain you should take proton pump inhibitor drug like pantoprazole or rabeprazole plus antacids. For replenish the water and electrolytes you should drink oral rehydration solution in stead of water. Avoid spicy and oily food, fast foods. Include curd in your diet. You can take probiotics to reestablish the normal bacterial flora after antibiotics use. Dehydration can lead to headaches. You ll be alright after adequate fluid replenishment. Hope my answer has solved your query. Take Care. Thank you.