What can cause severe butt pain?

Sciatica: When there is pressure on scitic nerve as it leaves the spinal column, there can be pain on one side of buttocks radiating down back leg. It gets better with walking. Therecan also be inflammation in sacro iliac joint.

Piriformis or SIJ: Piriformis is a muscle that frequently spasms with improper sitting positions or pelvic imbalance. Sij stands for sacroiliac joint which may become dysfunctional for the above reasons. Treatment is nsaids with exercises that address the dysfunction. Physical therapist can teach you those. Google psoas stretches and piriformis stretches.