Thanks for asking at Healthcare magic. I read your question and I understand your concern. Fluclixacillin doesn t cause pain in back nor shortness of breath. It causes pain only if it is injected and that is too at injection site. other side effects are GI upset, Urticaria, rash and hypersensitivity. You have develop GI upset and slight bronchospasm due to fluclixacillin I would like to advise you to take antiallergic like cetrizine, spasmolytics like Hyoscine bromide or drotaverine. Take plenty of water and fibrous foods like fresh fruits and vegetables and fibrocal. Hope it helps best. Don t forget to rate this answer and close the discussion at Healthcare magic. Take your best care. Regards Dr Ejaz