I'm 24 m. I have sciatica to my left leg. I feel somthg is moving at my lower left back and backside of left knee. Why movment at back of knee?

Nerve pain. Sciatica is an irritation of the sciatic nerve which is a nerve that goes down the back of the leg. You can have burning, itching or crawling sensations from your buttock all the way down the back of the leg.
Sciatica. Pain in the distribution as you suggested describes the pattern of the nerve that travels in the leg called the sciatic nerve. The sciatic nerve is not the cause of the pain source, it is the result of an irritated nerve typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease requiring further evaluation by a spine specialist.