In truth, most of his reaction to the pain came from the pure shock of feeling it on an entirely new metaphysical level. He could recover quickly enough, so long as he didn't get further disoriented. Like, say, flung violently across the stage, or grabbed out of the air with gravity magic.

By the time Sans' magic has turned him blue, confusion has given way to fear. Ideally it would then give way to anger, but instead it's spilling over into panic, faster than he can give it direction. There's too much going on, coming at him and closing in from every side and faster than he can register.

He yells out, desperate. Branches on bushes rattle wildly, leaves scatter and fly around the room, starlight flickers in and out of existence with crackling sparks as he struggles to escape what holds him: the blue magic, the darkness in his soul, the bonds of his label, his wild and disjointed feelings. His magic flails as he does, a flurry of aimless motion desperate to make it all stop.