Knee pain

I am relatively new to FB. I started off with the 5 day challenge for busy people, immediately after which I purchased the 8 week FB30 for busy people. So I have been working out FB workouts for the past 13 days to be exact. I was feeling great all these days. But from today morning, I have been feeling weak in my knee and it hurts to sit and get up everytime, and even to walk :(

What could I be doing wrong? I'm making sure I'm doing it the right way. Could it be nutrition related? Please help. I really want to continue working out.