She had started taking a few steps away after he had taken the towel. She hadn't expected him to go after her again. That was her main mistake. She wasn't expecting him to be just as ruthless as she was.
Bellatrix had turned towards him when she heard his footsteps, and had turned right into his fist. It knocked her back a few steps, and she hissed in pain. She was going to have a black eye now. "Alright, Albus. If you want to keep going, I'm not going to stop you." Right as she finished speaking, she came back up to him, feinting with her left hand as she took a step past him so that she could kick his legs like he had done to her.
When he was on the ground, Bellatrix went up to him and put her foot in the middle of his back, holding him down. "Do you surrender?" She was breathing heavily, and her eye was throbbing, but if he wanted to keep going, she was certain she could.