I had similar issues caused by a hypermobile SI Joint. A Dr. I found, after 4 years and 6 Drs., who diagnosed the lax ligament that connects the pelvis with the spine. A procedure called prolotherapy fixed the ligament and the joint now only moves slightly out of place, an issue I can fix with 'knee to the chest' twice a day, or if needed.
The original issue had developed after a hip operation that left one leg shorter than the other. The SI Joint accommodated that with its hypermobility, but then caused issues including semi-dislocation of my leg when moved in a certain position. Prolotherapy turned on specific high levels of self-healing for the over stretched ligament.
I am old and was always very flexible and athletic. I believe the muscles held my hypermobile joints in place for a lot of years. Finding a DR. familiar with EDS and hypermobile joints is difficult but Mayo does know. It could be your issue. I felt the issue in my groin before I found the answer.