From my experience with video analysis, I see you throwing entirely with your Upper Body. Stiffness after throwing comes from your motion's force being redirected into your Throwing Arm instead of ending up in your Throwing Hand.
As I view your motion, I see an extreme Pelvic tilt; notice how your Head moves to your Back Hip as you begin your motion. Your "tilt" forces you to lose your Trunk.
According to a Ohio State University Sports Medicine Center Study
, the more you stabilize your Trunk (the less you tilt your Pelvis) the more injury resistant your motion and the more you'll positively impact your overall pitching performance.
To stabilize your trunk, my recommendation is to begin your motion with your Front Knee in front of your Front Hip; not behind your Front Hip.
By changing your Front Knee position, you'll give your Core a chance to properly move the energy stored in your Legs directly into your Throwing Hand.
L.A. "Skip" Fast
Professional Pitching Solutions