The answer to this question can be found in chapter 7.

A piercing shriek suddenly reminded the Westing heirs of murder. While passing out the checks, the lawyer had stepped on Crow’s sore foot.

The heirs to the Westing fortune are being paired up and getting information about the "game." Crow is paired up with Otis Amber, and they move to table 6. Plum then reads the next rule, which is that each pair is to be given a check for $10,000. Plum begins to hand out the checks to each set of partners, he steps on Crow's sore foot, and she shrieks in pain. The reason that her foot is sore can be found in chapter 3. She has a corn on one of her toes, and Dr. Wexler cuts it out/off. The small operation leaves Crow sore, and she removes her shoes any chance that she can get. Her foot is already hurting from her minor surgery, and then the foot is stepped on. I imagine most people would cry out in pain.