Here's why she's crying:
1. In the back of her mind, she thought the old notion that men don't hit women would apply to her despite her being a cop; the shock that she was treated like a cop rather than a woman traumatized her.
2. After getting clocked, she expected a male saviour on a horse to come to her rescue and none did
3. She instantly realized that all her training in the gym was useless in the face of real danger
4. She got a reality check that any ordinary man is much stronger and faster than she is
5. She got publicly exposed as physically weak while in uniform and the embarrassment was eating her up
6. She thought that crying would gain her additional attention and sympathy, which it did. It's a card women play all the time in countless situations.
7. The event reset her back to her default setting as a woman to cry in times of uncertainty, fear, and confusion
8. The punch hurt a little