Pollen could have entered through yor intake or off your clothes.
It's well known to travel over a mile in the air currents
Or, you have a hermaphrodite that has pollinated herself due to genetics or stress.
If you only have only the one plant, then there is nothing you can do for her but stick it out.
If you have other girls in there, then you need to grab that hermie by the neck, cut her deep, and hang her high.