You want to be careful with that pain in your left knee, as it could get worse if you keep pushing it.
Dropping that right foot means that all of your weight is resting on your left leg, but it's probably bent in a slightly awkward position. That could place a lot of pressure on your knee, particularly the ligaments on the outside and front of the kneecap.
I would recommend trying a few simple exercises for your legs:
- Leg presses
If your knee doesn't hurt doing these exercises, it's defintely the result of your posture while on the board.