Woven fabrics are often stretched off grain when they are wrapped on a bolt.  You can easily check if your fabric is off grain.  First, one of the raw edges should be cut (or torn) straight.  To cut a straight edge, find a crosswise thread and cut next to it all the way across.  (You may be able to pull on a thread to make it more visible.)  Then fold the fabric in half on the straightened edge and align the selvege edges.  Pick up the fabric by the selveges and look at the fold.  If it's smooth, your fabric is straight.  If it ripples, your fabric is off-grain.  Slide the selveges until the fold is smooth, then straighten by pulling across the bias from the "long" corner all the way down the fabric - you may need an extra pair of hands.  Check again.