I discovered once, when the same thing was happening to me, that my fabric was 'drifting' to the left as I sewed a seam. The seam started out as being 1/4 inch, but then at the end of every piece, it was much narrower. This caused the curvy thing. I have learned to use my left finger as a guide to try to keep the fabric straight, especially when sewing HST's. Some people use stilettos for this.