Where you are pointing at is right at the level of your PSIS (posterior inferior iliac spine). That is the bony point you are referring to. Typically, pain in this region does indicate SI joint involvement. Compare it to the other one. Does it feel a bit lower? If this is the case you are out of alignment. This malalignment can cause gapping in the joint causing the nagging pain. Luckily, this can be fixed pretty easily by a muscle energy technique.
Try lying on your back and raise your left leg up to 90/90 hip and knee position while leaving your right leg flat on the floor. Once you are in this position, resist hip flexion on the left side to activate your hip flexor (push down on your left leg while maintaining the 90/90 position). At the same time, drive your right leg into the floor using your glute on your right side. The simultaneous activation of your left hip flexor and your right glute max will rotate the innominates of the pelvis. Do 3 sets x 5 seconds twice daily. Typically with muscle energy techniques, you won't feel instantaneous relief. However, usually by the next day it should feel significantly better if it is truly an SI problem. Good luck.