Why when I do a squat I can’t straighten my back properly?

I’ve been doing 50-100 squats in a row pretty much everyday for a month. After the first day of 100 my legs were killing me but I had DMS but it’s been good ever since (I usually put on some music to help me push through). Been trying to squat deeper and with weights to make it harder recently. However, no matter what I can’t seem to straighten my back like I see some people do in videos. Whenever I try, my back starts cramping. Any tips?