After 2 years of chronic pain I finally got some answers:
The MRI of my pelvis shows swelling in the tendon that attaches my left hamstring to my pelvis and in the pelvic bone where the tendon attaches!
By the way, I'm kind of being sarcastic about the significance of this. It's not surprising there is swelling in that exact area since that's where I'VE BEEN FEELING PAIN FOR 2 YEARS!!
I guess the good news is it tells us what it isn't: it isn't a tear or a fracture or a joint problem. It also meant that I am a good candidate for the cortisone shot. So I got a huge needle in the ass today. It hurt like hell, but I'd be willing to do it 50 more times if it will take the pain away permanently. Unfortunately, I can only get one more shot if this first one doesn't take and I have to wait a few weeks to see if it has been successful.
What bugs me is we still don't know the why. But the doctor said that given the nature and location of this injury it is very likely that as my pelvis shifted position to accomodate my pregnancy with Little A, this area became more vulnerable and I somehow injured it and once it became injured, it became continuously irritated by any pressure on the area (i.e. sitting).
Ah well, at least I now have ammunition to use for future guilt trips on Little A: Look at what I endured for you!!!