Knitting on circular needles and blanket turning out "u" shaped..? help!

I just started a baby blanket on circular size 10 needles. I decided to cast off to see how long it was and notice that my trial mini blanket has a definite U shape to it… what am I doing wrong?

Count your stitches, you may have added a lot more. Or it’s possible either the cast on or bind off edge was really really tight. Other than that, it’s hard to say without a picture.