It's piriformis syndrome.
Piriformis is a muscle which is present in the buttock region.
By the way, I would like to know if you have diagnosed this yourself or some Specialist has done it.
If you have done it, then its better you let the expert do the work. There are only very few ways to diagnose this condition. If a doctor has done it, then here are a few options.
You can take a pain killer along with a muscle relaxant drug. If its helpful for you, you can do this for symptomatic relief but this cannot be taken as a long term treatment. Therefore you need to consult a physiotherapist so that he will also examine you and suggest certain excercises.
If it still fails, then your only option would be to go for release operations.
Kindly consult a orthopedician for the same.