Sacroiliac (SI) Joint
There are two of them in your lower back, and they sit on each side of your spine. Their main job is to carry the weight of your upper body when you stand or walk and shift that load to your leg.It could be a dull or sharp pain. It starts at your SI joint, but it can move to your buttocks, thighs, groin, or upper back. Sometimes standing up triggers the pain, and a lot of times you feel it only on one side of your lower back. You may notice that it bothers you more in the morning and gets better during the day. It is quite common and often confused with hip or lower back pain.