Hi ! You have not mentioned the site of pain in your stomach/abdomen. However, the way you have described your symptoms and signs, possibly you have a sub acute attack of appendicitis, where you got nausea with mild fever which caused you a headache. You did not have a diarrhea, which is not favouring a food poisoning. With all probabilities, the sub acute attack has caused the omentum to seal it off with a localised inflammation of peritoneal cavity and a appendix mass formation, which gave you a weired feeling on touching. This can give rise to pain and tenderness on moving, coughing or sneezing or any work where intra abdominal is altered and the inflamed perietal peritoneum is distubed. In my opinion, you should take the opinion of a general surgeon by getting clinically examined in person, who may also advice you for some blood and radiological investigations like ultrasound abdomen. If it proves to be appendix mass, then you may have to be admitted in the hospital for IV drugs, and other conservative management, and the surgeon may advise you for an interval appendicectomy. Get yourself seen and investigated by your doctor at the earliest possible. Good luck.