Hi.. Welcome to HCM.. Flucoxacillin is an antibiotics and many a times taking antibiotics can cause Gastritis and acid reflux leading to heartburn and pains.. Certain foods that are that can exaggerate the condition along with antibiotic intake are spicy and heavy meals, frizzy drinks, chocolates, onion and garlic, citrus foods and fruits, tea and coffee, alcohol etc.. I would suggest you to start taking Proton Pump Inhibitors like Omeprazole or Pantoprazole empty stomach in the morning till you are taking antibiotics, antacids like Milk of Magnesia can be taken half an hour before meals.. You should take more of probiotics like Yoghurt in diet.. Drink plenty of water.. You should avoid large meals and take small frequent meals.. You should take walk after meals.. Sleep with head slightly raised as compared to feet. You should do regular moderate workout.. Avoid stress.. If there is no improvement you should consult your Physician and get evaluated and he can either change or stop the medication or change the antibiotic if further dose is needed.. Hope this helps.. Regards..