@tennis123 The symptoms you are describing with your pelvis sound like the pelvic alignment may be off and causing nerves to be pinched. My pelvis has been able to shift, and on one occasion and cause some vibration feelings on half of my groin and I think that is from the Pundental nerve. It was easily correctly by physical therapy. I have been working with a physical therapist and sometimes my ilium bones are inflared which causes irritation to the colon that sits in front of it and that can cause constipation and pain in the GI tract. If the hip flexors are too tight, it can cause this by pulling the illium forward and inward or they could twist up and downward. When I feel this coming on, I stretch with a lunge type stretch to stretch a leg backward as mush as I can, and do this while I have a tennis size ball under it putting pressure on the tight muscle to stretch it. Sometimes I do this against an open door frame while standing. It hurts to do that, but it opens up the constriction. I also do a stretch my physical therapist showed me on resetting the pelvis that involves opening my thighs against resistance, and then squeezing my thighs together against resistance, and sometimes I feel it click into place and it may or may not make a clicking sound.. The pelvis has 3 bones on each side and is jointed, and sometimes it can kind of twist out of alignment. When that happens, I get pain one one side of my lower back or sciatic pain down my leg. The tailbone can even be pulled out of alignment.

These types of problems cause symptoms similar to spine problems. The muscles connecting the pelvis to the spine can be at fault by being too tight and putting a lot of pressure on the spine.

This article explains why this can happen and it's called Lumbar Plexus Compression Syndrome because it involves nerve roots of nerves going to the legs.

"How to identify and treat lumbar plexus compression syndrome (LPCS)"
https://mskneurology.com/identify-treat-lumbar-plexus-compression-syndrome-lpcs/
I know this article is very technical and discusses how muscles and nerves intertwine. From what you said, it sounds like you are very tight through the pelvis and lumbar spine as you said sitting down causes a lot of pain. Have you been evaluated by a physical therapist? I think that really could help you particularly if you find a PT who also does myofascial release. The fascia has to glide to allow the body to move, and it you are stuck, it will just increase pressure on other things instead of gliding as it should.

You may want to look at our discussion on Myofascial Release.
-Neuropathy - "Myofascial Release Therapy (MFR) for treating compression and pain"
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
I have done a lot of this MFR work and it really helps me. It gets the body moving and aligned as it should be and releases tight stuck dehydrated tissue. You don't need a specific diagnosis to do this, probably just a script from any doctor about working on pain and the structural weakness that is causing it. You can search for a provider at http://mfrtherapists.com/

It sounds like you have ruled out a lot of other structural issues. How is your posture? That can lead to getting stuck if you are always in the same position, have bad slouching posture, or may be sitting too long and too much of the time. Walking is great to maintain pelvis alignment and strength and so is horse back riding at a walk which builds core strength to maintain and strengthen the spine pelvic alignment. I have a horse and this helps me. I am a cervical spine surgery patient with a fused C5/C6 and building core strength from below gives me a strong foundation to support all of my spine and head on the column of vertebrae.

Are you involved in any sports that could have led to this issue of tightness? Is it a lack of exercise or lack of regular movement causing this? Are you sitting too much engaging in online activities that affect your posture?