What presser foot are you using. If it is not an even-feed or walking foot that may be your problem. A regular foot will send the two fabrics under the needle at a differing rate. One is pushed and one is pulled. On short seams this is not a problem, on long seams it is. If you don't have a walking foot, try this trick when sewing the seam, hold the two fabrics together in you left hand up a little and somewhat taut. This is a trick used by seamstresses in clothing factories. Another is to sew the strips alternating top to bottom, bottom to top, that way the distortion cancels each other out.