Which Injection Is Given For Treating A Knot In Hip Muscle?

My doctor told me today I have a knot in the muscle in my left butt cheek or under the muscle. I was in so much pain after he pressed on it I can't remember what he said exactly. Except something about an injection guided by xray and the sciatic nerve being compressed. I'm in agony from my toes to my lower back. What exactly is this injection he's talking about doing?