A lot of fabric does stretch a bit one direction (along the width) and not the other; it's just the nature of woven fabric. But usually it's not enough to throw a seam off, at least not by much.
I'm not really sure what could be making that happen, but someone will come along who does.
Oh - you aren't tugging or dragging on the fabric as you sew, are you? I had a bad habit of tugging the fabric through the machine when I was a beginner, and that would sometimes give me weird stretching & curving.