Rotation facing the right way problem

I have this whale and I would like to have it rotate by its head. It has a velocity. Basically what I am doing is every so often the whale gets a new velocity ( I store the old and compute a new)

I am trying to rotate by means of the following

float diserAngle = Vector2.Angle(oldVelo, newVelo) ; Vector3 vec = new Vector3(0,0,diserAngle);

iTween.RotateTo(gameObject, vec, 2F); oldSpeed = newVelo;

It seems to only look natural when going to the left of the screen. When it needs to rotate towards the right of the screen the whale looks like it does not rotate and wadles backwards.