Clicking knee

Hi,

I took an intense step aerobics class today. Afterwards, I noticed that my left knee clicks whenever I climb the stairs or extend my left leg. My left ankle/foot hurts too. The knee hurts if I sit with my knee bent for a while. It is very difficult to sit on the floor or ascend a flight of stairs.

What do I have and what is the proper course of treatment for it? Should I visit a physical therapist? Or just wait a few days and back off from the gym? How long should I refrain from exercise?

Thank you very much!