No, it curled because when you do all knitting in the round it becomes stockinette. The reason for purling on flat knitting is to get back to the other side without the bumps caused by all knit. Stockinette fabric curls because the stitches on one side (the purl side, I believe) an infintessimally smaller than the stitches on the other side. This means that they pull on the fabric, and make it curl. To prevent curling, you can do a few rows of garter stitch which is the same on both sides.
To do garter stitch in the round, you should mark the beginning of your round and knit one round. When you come to your marker again, purl that round. Repeat the knit/purl alternation for about 5 rows or so and it will create a band of garter stitch that will prevent your hat from curling. There are other ways to do this, but this is probably the simplest I know of.
I'm a boy, by the way.
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