My leg is slipping FORWARD?

When I've been riding lately, my leg keeps slipping foward onto the girth instead of right behind it! I'm not pushing it foward at all! And when I'm cantering I can feel that its on the girth and is sliding! Do you think that its my half chaps because I noticed that on a large area of them, the suede is all worn off, leaving a very smooth leather area. Help Please! I have a double judged show coming up and want to do good! Thanks!