Use the link below to look up your Ford Mustang's smog check history:
Vehicle Smog Check History
It is possible the Catalytic Converter on your Ford Mustand is defective however you'll need a CAT intrusive test or HC Snap Test (both performed only by a smog check repair station) to verify this.
It is not common that a vehicle passes the smog test and 200 miles later develops a check engine light fault, but it is not impossible either. If you feel something "shady" you can always contact the Bureau of Automotive Repairs Investigation Division at (800) 952-5210.