It could also be a form issue. Are you pushing your knees out as you squat down? Squats are all about sequencing. First, the butt starts to go back, then as you continue to sit back the knees push out and bend to allow the butt to move downward. If the knees don't go out, then there's no room for your stomach to go between your legs, which forces you to bow your lower back to continue to lower the weight.
Current PRs at 242- 495, 345, 585, 1425