Does performing an act against my will, under duress, expel me from the fold of Islam?

You are suffering from obsessive thoughts in this regard, and a Muslim does not abandon their faith except by committing acts that necessitate apostasy. What you have mentioned does not necessitate disbelief, but if it was a sin, then you have sinned, and you must repent and seek forgiveness. Do not return to such acts, and cast aside these obsessive thoughts.