data on the page

Hi expert, I have a drop down list with page size 10, 20, 50.  When it is on page size 10(default), it is weird that after clicking the second page link (2, 3..), the retrieved data will go to the following code. How to fix it. Appreciate it!
If Not Page.IsPostBack then
   BindData()
  End If 
 End Sub
 
 
 Sub BindData()
 
  
  Dim strSQL
  
 strSQL="SELECT * FROM  products  order by productsku"
...