Checkboxlist problem

Question

I have a checkboxlist with 3 checkboxes - whose values are "S", "J" and "Q"
i needed a way on the server side i.e. on the chk_SelectedIndexChanged(object sender, System.EventArgs e) event
which would let me know, as which of the 3 checkboxes got selected or unselected.
the following code would give tell me the checked-unchecked state of the 3 checkboxes but wouldnt tell me, as which of the got clicked --
 ListItem lstSelf = chk.Items.FindByValue("S");
 ListItem lstSelf = chk.Items.FindByValue("J");
 ListItem lstSelf = chk.Items.FindByValue("Q");

Please help