Re: Lowe back, spine, & s.i. joint
Unfortunately any high impact event such as a motor vehicle accident seems to bring on osteoarthritis in the spine much sooner than it might develop otherwise. I'm afraid this is one thing you will be dealing with into the future.
I don't know what type of doctor you are currently seeing for your back, but be sure you are going to a fellowship trained spine surgeon. This can be either an orthopedic SPINE surgeon or a neurosurgeon who limit their practice to issues of the back and neck. They are the two specialties that have the additional training and knowledge to properly diagnose and manage patients with back and/or neck problems. All other specialties may be able to treat you, but you will get the most accurate plan of treatment from a spine specialist.
Did you have a large baby or a particularly difficult delivery? Your pregnancy and delivery may account for the problems you are having with the SI joints. Sometimes this is when SIJ problems begin and they can plague you for the rest of your life.
The SI joint in normal functioning moves just a tiny bit. When the spine is out of alignment, or when we vary our posture and structural alignment, the SI joints (one or both) can be affected. It is not unusual to compensate for back pain by changing the gait you use when walking, or sitting differently, etc. This can put additional pressure or stress on other parts of the spine. Muscles, for example, will tense up to "guard" the injured area, ligaments will shorten or tighten for the same reason. This usually results in muscle imbalance and can be enough to pull the SI joint out of position. It may be that you are carrying one hip higher than the other, or tipping your pelvis further forward than is optimal.
Unfortunately all this structure is interconnected and by changing something at one level, it can have a negative effect on something at another level. For a couple years I had to have my SI joints regularly adjusted. I would develop what felt like "hip" pain and it made walking difficult and painful. We eventually figured out it was all due to the hypermobility of my left SI joint. There are a variety of ways this can be treated if it turns out you really have a problem with the SI joints.
When it feels like you are popping it back into place by moving your leg, that's exactly what you may be doing. It is not difficult to learn an exercise where you can "reset" the SI joints.
Generally speaking, people with spinal stenosis get some relief when leaning forward...as it opens up the disc space and gives the nerve more room, not less.
I suspect you have a bulging disc or something similar that is pressing on a nerve. When you make a sudden move it is "pinching" off the nerve, causing that temporary numbness.
I think you need to see another spine specialist for a second opinion. Be prepared to be dismissed, to not have your pain taken seriously, due to your age. You may have to stand your ground and calmly explain not only that you are in pain, but give examples (like bending forward to wiping baby's face and developing numbness).
Take some time to make notes prior to that first consultation. Keep track of what makes the pain better or worse; what position, like sitting, standing, walking or lying down, makes it better or worse...the quality of the pain, like numbness, electric shock, tingling, feelings of cold, etc. And be sure to mention that you were involved in a MVA when younger, but it was never fully investigated. You may have fractured something that has subsequently healed but is still causing some issues.
Also have them check for spondylolisthesis. This is done with a flexion/extension X-ray and checks for instability. This is one thing that does occur in the yoounger population and is easy to look for. It can cause all the symptoms you have described.