I'm no expert (far from it in fact), but the most obvious thing to me is the bar position on your back, or rather in this case, on your neck. Presumably you are trying to do a low-bar squat, but you have the bar positioned for a high-bar squat. You need to move the bar way down your back.
Watch these two videos and you'll see what I mean:
The Squat - Bar Position with Mark Rippetoe
How to Low Bar Squat With Mark Rippetoe | The Art of Manliness