I had the EXACT same thing happen. And ironically enough, I was also in the military.

What I gathered was that I was INSANELY tight…everywhere. You just have to keep doing hip, knee and ankle mobilitywods. Eventually it will loosen up so that you can do it with the shin vertical. I also had problems if I ever tried to sit back on my feet on the floor. I could never get all the way back, and if I tried, I would feel a similar pain that you describe.
I know it’s not direct advice,but you should know that you’re on the right path.