Si-joint pain (over 6 years now, doctors didn’t find anything)

My right sacroiliac (SI) joint has been painful for several years. The pain comes and goes in waves — sometimes it gets worse, and sometimes it improves. More recently, this type of pain has spread to my entire lower back as well.

The pain is dull and constant, and it gets worse with movement. Twisting my lower back seems to cause the most severe pain.
When I lie on my side, placing something between my legs provides some relief.

I have had X-rays and an MRI, but neither showed anything abnormal.