Dynamically get Modified fields in trigger

We have a case with 100 fields if i updated 1 or 2 fields, to know that field iterating all 100 fields. How to get that field without iterating 100 fields.

Below Trigger was implemented

trigger fields on Case (before update) {

Map<string, Schema.SobjectField> caseFields = Schema.SObjectType.Case.fields.getMap();
case caseold = trigger.old[0];

if(caseold.get(fieldName) != null)
For (Case cs : Trigger.new) {

    for (string fieldName : caseFields.keySet()) {

        if ( cs.get(fieldName) != Trigger.oldMap.get(cs.id).get(fieldName)) {
            string oldvalue = string.valueOf(caseold.get(fieldName));

            string newvalue= string.valueOf(cs.get(fieldName));

            cs.IterateField__c = 'Changed '+fieldName +' From '+oldvalue +' to '+newvalue;
            system.debug('Changed field values:'+ cs.IterateField__c);
        }  
    }
}

}

But, faced issue those trigger for iterating all fields. I want to iterating only Modified fields.anyone help me