Re: SI joint and overdeveloped hip flexor. Low back pain.
The SI joint doesn't "come out". It barely moves, as in millimetres. Recommend mobility, soft tissue work on your hip flexors (some very thick and hard to elicit length changes, but some responsively tight and easy to solve, such as (ilio)psoas) and continual stretching. By all means strengthen posterior chain, but don't neglect the cause