Is this the stitch you cast on first? If so, it may be caused by your knot.
For the longtail cast on, you don't need to make a knot, just hold the wool over the top of the needle with a finger on your right hand (if the loops are on your left).
If you still want a knot, or when you make one for another caston, you need a slipknot. You can find directions on the net easily, if you're not sure. When you make a slipknot, pulling on one strand will make the knot tighter, and the other strand will loosen it. MAKE SURE that it is the short tail that TIGHTENS the knot, and the end going to your ball that loosens it, otherwise you will get a sloppy loose stitch.