Why is my alignment off?

I noticed that after I take the picture and the film fully develops that the alignment is off. Im not sure if thats the right word to describe it. The film is more towards the left of my shot. I was taking a picture of my books and when the film developed a quarter of the books were cut off on the right. I tried to take another picture I thought maybe I just slipped or something but the same thing happened again. please help im just a girl trying to take a picture.