I remember your post about your gigantic follie last month! You are on
Femara now, right? You said Clomid first, then
Femara, but I think I remember you are on Femara.
Anyway, a trigger shot is called such because it "triggers" ovulation. Thus, your doctor is inducing ovulation before your body actually does it naturally. With fertility issues, many women (including me) do not ovulate normally on their own all of the time, so that may be why you did not last cycle.