Consider to negate the output of the PRiSM (which you hopefully use). Can you please post your complete project, so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
Check pin output state on program startup. I suspect it is HIGH (for some 100ms while program initializes). If so, change pin startup state in pin properties.