warm up, warm down
stretch, dance
give a day to rest
and heal
balance

There are times in a person's life when they would be ill-advised to do any form of exercise. YOGA was developed after observing the activities of certain animals and they noted that for example cats/felines rarely exercise but perform stretches combined with breathing techniques and certain limb movements - essentially, this represents the starting point and the end.
performing an exercise properly relaxed once should be enough. avoiding any adverse strain. remain relaxed throughout and the exercise becomes easier.
but if the body is afflicted by some planetary tranisit then it becomes difficult.
for example saturn which rules the bones and sinews can often make you stiff.
usually good aspects from either neptune or uranus will benefit yoga practice. but if they planets are by tranist progression or natally afflicting an individual they might find it difficult to pursue their goals.

Stretching beforehand is important. Could also be diet/nutrition related. I know when I don't get enough calcium and magnesium, it affects my leg muscles and I get cramps. Be sure to drink water as well.

I'd say talk to your teacher about this but also ask your doctor if you haven't already about what movements help your condition and which worsens. Whatever is wrong with your knee or hip should be diagnosed by a doctor rather than saying "i think it's the sciatic nerve" It sounds like to me that you may have pulled a muscle in your hip which would effect how the knee feels. But it's best to know for sure. As for feeling drained and shakey in your legs seems to be that your over working the muscles there. If you are doing too much physically this can open you up to injury. Take it easy until you know and until after you heal.