Why do I always get butt pain when I am sitting?

Back as source. You might have your back checked by an orthopedic specialist. I am a runner and had that problem in my butt for a long time. I had back surgery (l3-l4) for a herniated disk in 2011, and ever since i don't have the butt pain anymore. I never linked them at that time.
Sacroiliitis. Inflammation of sacroiliac joint due to pelvic imbalance when sitting or psoas muscle spasm. Try psoas stretches and see if it helps. If doesn't, see a pain physician.