This is a 2 part question:

I started doing front squats with kettle bells. When I upped the weight, when lowering my body I can't breathe in a full breath of air. There's no room in my lungs. Is this simply pressure being put on the trunk of my body only allowing for a smaller breath?

I noticed that when I upped the weight, I would also be hitting my lower back muscles. I never had that with standard squats, or at least not as noticeable. I had people checking my form and there doesn't seem to be a problem. Is this normal?