Ouch!
It does sound like sciatic nerve irritation. That can come from a disc putting pressure on the nerve root or from a muscle called the Piriformis, spasming and putting pressure on the nerve itself. If you also have sacro-iliac pain, it might be the latter.
If you can, maybe try massaging the upper buttock near the sacrum to see if it brings relief.

The piriformis syndrome
http://www.rice.edu/~jenky/sports/piri.html

With love, Erika