jQuery Grid Plugin - jqGrid

Hi, I am using the jqGrid for ASP.NET WebForms and I need to know if a cell or a row had changed. Therefor I tried to call the function getChangedCells('dirty') but it always return an empty array

Here is the code where I use the funciton:

  function editRow(id)
             {
                 if (id && id !== lastSelection)
                 {
                     var grid = jQuery("#<%= jqGrid.ClientID %>");
                     grid.restoreRow(lastSelection);
                     grid.editRow(id, true);

                     var changedCells = grid.getChangedCells('dirty');

                     lastSelection = id;
                 }
             }

I really appreciate any help.

Thanks