It seems like your grip could be more secure coming out of the rack. You're moving the bar an awful lot before you start your first rep, and it looks like you may be intercepting the weight with your hands. Could be the reason for your discomfort. As for the squats themselves, the bar path is forward of the midfoot, watch your heels rise off the floor. Think about the master cue. Your upper back seems to round as well, possibly from not having it racked securely, or just from you not thinking about keeping your chest up.