Shape tweens on lines trouble

1

I’ve done plenty of websites in flash, but this time, I’m trying my hand in animation. I’m just doing stick people: a line for each arm and leg on it’s own layer. I bend the lines and move the ends with the the selection tool and I put a shape tween in. It works only half the time. Other times the ends of the line will swap places and the line curls around. I don’t see why it would do this since I only moved a specific point. I’m a shape tween noob, is there any way around it?