riggeri am testing a trigger.i am calling this method

  GMirrorUtil.createTimeLine(Trigger.new, contentMap);

My create timeline function is 

public static void createTimeLine(List<sObject> objList,Map<String,String> contentMap){ 
      Map<Id,User> userMap = new Map<Id,User>([Select id,Name From User where Authorize__c = true]);
      Map<Id,GlassUserApiSettings__c> userSettingsMap = new Map<Id,GlassUserApiSettings__c>();

       for(Id user_id : userMap.keySet()){
         GlassUserApiSettings__c userSettings = GlassUserApiSettings__c.getValues(user_id);
        if(userSettings == null)
         { User u =userMap.get(user_id);
            u.Authorize__c = false;
           update u;  
         }
          else userSettingsMap.put(user_id,userSettings); 

       }
       for(sObject obj : objList)
         for(Id userId: userSettingsMap.keySet() ){
           String type = '';
           if(contentMap.get('Object').equals('FeedItem'))
             type='Post';
           if(contentMap.get('Object').equals('FeedComment'))
             type= 'Comment';
           GlassUserApiSettings__c userSettings = userSettingsMap.get(userId);
           if(((String)obj.get(contentMap.get('Content'))).contains('@'+userMap.get(userId).Name)){
             Datetime tokenExpiredTime = userSettings.LastModifiedDate;
             tokenExpiredTime.addSeconds(Integer.valueOf(userSettings.ExpireDuration__c)); 
             String body='{"html":"<article><section><div class="text-auto-size">'+type+':'+
                         '<p class="yellow">"'+obj.getSObject('CreatedBy')+'&nbsp;'+obj.getSObject('CreatedBy')+'</p><p>'+obj.get(contentMap.get('Content'))+'</p>'+
                         '</div></section> </article>"}';
                         System.debug('Body is '+body);
             if(tokenExpiredTime >= System.now()){
               GMirror.TokenResponse res  =  GMirrorUtil.refreshToken(userSettings.RefreshToken__c);                 
               userSettings.RefreshToken__c = res.refresh_token;
               userSettings.AccessToken__c  = res.access_token;    
               userSettings.ExpireDuration__c = Integer.valueOf(res.expires_in) ;
               update userSettings;
             }
             String timelineRes = doApiCall(body,'POST','https://www.googleapis.com/mirror/v1/timeline',userSettings.AccessToken__c);
             GMirror.TimelineResponse createdTimeCard  = (GMirror.TimelineResponse) JSON.deserialize(timelineRes,GMirror.TimelineResponse.class);   
             if(createdTimeCard.id != null)
               System.debug('created timeline card :'+createdTimeCard);
             else { try{
                        throw new GMirror.TimelineException(null,timelineRes);
                       }
                    catch(Gmirror.TimelineException e){
                      System.debug(e.getMessage());
                    }
                  }
           }
         }          
    }

but when i initialize prepare String body='{"html":"'+type+':'+ '"'+obj.getSObject('CreatedBy')+' '+obj.getSObject('CreatedBy')+'

'+obj.get(contentMap.get('Content'))+'

'+ ' "}'; i check it in debug logs its giving me null for getSObject('CreatedBy'); i inserted it in my test class thats why trigger invoked and i am using afterinsert event in trigger . can any one please tell why its giving me null ??