[FONT="Comic Sans MS"][COLOR="#300090"]I agree with catlvr, you didn't do a double crochet. But I also don't think that crochet will prevent the curling of a stockinette stitch.
I can only think of one stitch that might stop the curl; alternate between back post dc with dc across/around. This would give a ribbed appearance.
Turning your work and working from the wrong side using a double crochet (dc) may have a better result.
Or using an afghan hook (from the wrong side) and the afghan stitch or tunisian simple stitch (tss) may help correct the curl. At least the tss curls for me much like the stockinette is curling on your project. By working from the wrong side the curling would be counter to what you have.