That illness really hurt Lesnar, he was never like that before. He used to take punches fine before that illness got to him, he took a massive knee in the Mir fight and was fine. Whatever happened, it obviously hurt him psychologically and his confidence.
This isn't the same Brock that entered the UFC. Now whenever he takes a punch, no matter how big or small, his immediate reaction is to backup, and turtle a bit. If you look at him before he was diagnosed with diverticulitis, he had no problem standing there and taking a few punches. Granted, the level of competition kept going up, but thats besides the point. Its a reflex of Brock's now, where as before he didn't have this problem.