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A little background… The hip flexors while they aid in moving the leg are also the strongest support muscle for the hip. They are so powerful that they can literally wreck havoc in the lower back. That referred pain is actually indicitive of a pelvic tilt. Mine is in left hip, and my left lower back gives me trouble, which is tied to my lower ribs. Kind of interesting. Anyways ur hip flexor is so tight that its tilting ur pelvis. Sounds bad but its treatable.
I do two things and for the most part my lower back gives me no problems. I do them before I goto bed at night. On the affected side put that foot on a chair, lean back and put that sides elbow into the crook of the hip where the hipflexor joins the hip and dig at it with your elbow. Work it from the inside out. If you feel your femur start to move stop and back off. If it thunks out of place then your gonna be sore for a few days. Next with that same foot up on a chair bend down and try to get that sides shoulder down to ur knee. I like to set a 45lb plate next to the chair and use that to pull down on. You also need to couch stretch and be able to hold a squat for 10 mins. But get ur scar tissue and pelvis straightened out first.
That alone will not stop the pelvic dysfunction. You will have to throw some money at this. I spent about 500 dollars in deep tissue massages. I went to a really strong masseuse and told them i wanted them to work on my scar tissue in the lower back. They use deep tissue massage and you will be sore for about a week, tip them well, its hard work. It took 3 sessions before my back was good. You also need the masseuse to release your psoas. Next thing is you will need to find a good chiropractor to set ur hips till ur muscles settle down. Once you get ur hip range of motion good, and ur hip flexor settled down and your pelvis centered you will be good.
Now, if messing around with this your back and hip will snap, crack, pop, and creak. that is all fine. thunks are bad. thunks mean u need to get to a chiropractor. Good news is this is about a 3 week process and with minimal maintenance you will be pain free.
EDIT: just fyi, im not a doctor or a therapist or anything, just someone who has delt with a lot of back pain in my life.