Flucloxacillin is very good antibiotic and sometime may cause stomach upset
But you should not take antibiotics just like that.
Get yourself examined by ENT specialist
and get done CT scan of nose and paranasal sinuses
to know the disease extent and then take nasal sprays so that whole body is not exposed to medicines again and again and if required get surgical treatment
You can use normal saline nasal sprays its harmless and can be taken for long time and will keep your nose clean and will prevent infections and allergies .