First Warehouse job and it hurts.

So I'm 19 and ever since I was 16 I've had fast food jobs. Well I started my first warehouse job distributing books. So I have to cut, lift ,and place heavy boxes. Well I cut with my left hand. Just this whole thing is really taking a toll on me. I've been working there a little over two weeks and my body just isn't handling it well. I'm literally having muscle cramps and everything burns. I can't move without something tensing up or having a cramp. Any suggestions on how to help my body get use to this kind of work and movement? My body literally feels like it's made out of cement