What is this pain in the back of my right thigh?

Hamstring? sciatica?: Commonly pain in the back of thigh is due to hamstring injury or sciatica. The latter may radiate pain past the knee and into the leg sometimes with numbnes. Sciatica is due to low back issues such as arthritis or disc herniation compressing the sciatic nerve/branches. If symptoms persist or you develop any weakness/paralysis in your lower extremities then seek attention immediately in an er.