Anal Fissure. Anal fissure came back?

Having functional bowel disease. I rarely get hard stool,  most of the time stool semi solid having no proper consistency and formation,  but just for one day little hard stool I get anal fissure. So I consulted a colorectal surgeon who told me after this surgery you will not get anal fissure for occasional hard stool and the surgery was Lateral anal sphinctertomy and it was done . Then 4 years after surgery I did not get hard stool for not a single day. But 2 days ago I got fever and taking antibiotics stool became hard and again anal fissure occurred. So, my question why after surgery just for one day hard stool anal fissure came back ??? It has no permanent cure so that for one time hard stool it doesn't occur .??