That's hard to tell where you "went wrong" without actually looking at the fabric, pattern, and quilt top, there are so many possibilities:
* The pattern could be wrong. Sometimes the designer will post errata to their website (if they have one), can you check?
* If you prewashed the fabric, it may have shrunk
* Your cutting or your rulers may be off, especially if you're measuring your cuts with your cutting mat and not your ruler
* Your seams may be inaccurate
* A combination of these factors, or others I can't think of at the moment....
How experienced a quilter are you? Can you post a picture of the quilt, and also the name of the pattern, or a link to the pattern?