That's the issue then, your character's shield is attached to her forearm, if you go through the motion of drawing a sword you see that it would be impossible to do that without your shield being moved out of position.

Unless she's wearing her sheath around her stomach, which would be highly irregular. Only Samurai spring to mind in wearing their sheaths in such a way that they aren't drawn from the waist.

However, despite me personally finding your description of disguising drawing the sword impossible, I went with it. Sigurd only exploited that specific opening on side-stepping, before that he intended to strike at her right side.