It was so big and full of people that she felt very emotional.

emotional 是形容词，来自名词 emotion（情感、情绪），加上后缀 -al 表示"与...有关的"。emotional 在这里指"情绪激动的、感情丰富的"，描述因外界刺激而产生强烈情绪反应的状态。