Question about squat form and unintentional "shifting" to the side at the bottom

When I get to the bottom of my squat, I tend to "shift" slightly to the right. I feel like I'm standing with my feet at proper width and all, but when I get to the bottom of my squat (especially under more weight) I "shift" a little to the right; not a lean or a fall, but as though when I settle into parallel position my whole body slides to the right a little bit. I can feel it in my left knee. Is this a mobility issue? Any tips?