Somehow, Elphaba managed to keep herself from fully realizing what she had just done. If she'd been in her right mind, she would have instantly stopped hurting Galinda. But then Galinda was still moving towards her. This confused her because any normal person would've gone in the other direction. Preservation of life and all. But instead, Galinda came closer to her, hugged her. Elphaba flinched, much like she had back at Shiz every time the blonde would hug her.
"No, don't touch me! Get off me!" Her tone had a slight touch of desperation to it. It was a sign that the real Elphie was still there, buried underneath the pain and rage. She was starting to respond to Galinda, and the wicked witch wasn't having this. She still had things she needed to do, a plan she had to carry out. There was no Wizard she had to fight against here, but there were others she could fight, that she would have to fight if she remained in control.
So in her desperation to get Galinda away from her, she cast the first spell that came to her mind. It was the fire spell. It gathered in her hands and she grabbed onto the blonde's upper arms, burning them and trying to pry her off of her. "Let go of me!!" All the while, she was trying to pull herself out of the blonde's grasp. She needed to get away, make Galinda go away. It was the only way she could get out of this.