Everyone had best sit down because I am going to (sort of) defend Trump here!!!
Although I haven't done a forensic analysis of the exchange (and I've only seen that clip and not anything that took place before) it looked to me as though Trump didn't like her question and so did his default look over there/blame someone else routine, intending to move on quickly to the next reporter. I wouldn't be at all surprised if at that point he had no concept that his telling her to "Ask China" could be construed as personal and relating to her appearance. Then of course things escalated as she stood her ground and Trump seemed to become even more erratic in his exchange with the next female reporter that tried to ask a question before he promptly ran away.
Maybe it genuinely felt racist to her, or maybe it was simply just too big a gaff for her to let it pass. We know that Trump hates it when people challenge or stand up to him and he appears to be so insensitive and lacking in empathy that I can believe he didn't initially grasp what he done. A normal person would have apologised for any misunderstanding and clarified their point, but not Trump who seems to have no ability to think on his feet. I can imagine a handler around the corner realising that Trump was losing his grip on proceedings and frantically telling him over an earpiece to walk away before the situation span completely out of control.
Not only for you but for others that reads your post as well. Here is a clip of said conversation between Trump and the reporter, showing her as well.