If I tripped and twisted my knee and heard a loud popping sound, does that mean I tore my pcl?

Possibly. Assuming that the knee swelled as well, usually that scenario suggests either a meniscus tear or a ligament injury.
No. It is unlikely that the PCL would tear with a minor tripping episode. It is usually torn with a hyperextension injury or with a direct blow or fall on a flexed knee. The important thing is whether or not your not your knee is still symptomatic. If its been a few weeks and the knee is still feeling loose or unstable, you should probably have it checked out by a trained professsional.