What batting did you use? And how close together were the quilting lines? Ordinarily moderate quilting will prevent fabrics from distorting due to uneven shrinkage because at that point the batting controls shrinkage. If you tied the quilt or had quilting lines quite far apart, that is likely the cause of the problem. You may be able to fix the quilt by getting it thoroughly wet and blocking it to size. Once dry, you can add additional quilting to the blocked quilt.
Washing a quilt top before sandwiching is actually riskier because there is no quilting to prevent the uneven shrinkage and you can get a lot of frayed seams.