You pull open your trunks to see that...you have a vagina! Your stunned, that strange feeling was more then just shrinkage it was you growing a vagina.
While you were swimming your body was 'repairing' the damaged parts of your body and apparently this was it's first step. You quickly stop looking and close up your trunks - "the doctor was wrong, I'm doomed to change into a woman and it's only a matter of time before everything else follows suit." you gasp quietly.