Stockinette stitch problem

I've looked through all the library books I can find and cannot find a similar problem. Every time I knit in stockinette stitch my knitting pulls down to the bottom left hand corner. So the knitting is longer on that corner but also the whole body of knitting seems to gradually move toward this point. I don't know quite how to explain this problem any better. I am a new knitter and feel quite disappointed about this. Is there anyone who can help me with an answer?