Hmm. Too low as mg said and maybe too bent. Did you bend your elbows so far that the bar went under and beneath your head? That might hurt.
I've been doing tons of these, but closer in form to a pull over. I don't bend my elbows much more than 45 degrees. If I do them with the straight bar I get pain in a variety of joints, especially the wrists.
I found that using something close to a neutral grip on the EZ curl bar is best. I'd use a triceps BLASTER!!!! if I owned one. (I might buy one of those bars with neutral grip handles in the middle. . . ) I don't know if it will help your problem though. In the video, Rip uses the opposite part of the ez bar than I use. It's not clear to me why. I pick it up like I'm doing something close to a reverse curl.
Using a dumbbell held in both hands feels the best, but I can't get enough weight this way to make it meaningful except at high reps.