To feel leaden is to feel heavy burdens due to strong emotions.
To feel leaden is a figure of speech. Leaden is a metaphor for emotions. Metaphors compare unlike things. Lyddie is not really full of lead.
When Lyddie feels “leaden” it means that she feels very heavy or burdened by her emotions. For example, when she talks about her home and family, she describes herself as feeling “leaden with sadness.” This means she is so sad that she feels as if she cannot move, like she was full of lead.
"There's no hurry. Wait till your family comes. I don't know when my brother and I can ever get back." She felt leaden with sadness. (Ch. 6)
This incident occurs when she gives Ezekial money to help him run away. He tells her that she needs the money and should not give it to him, and she replies that she got it from selling her calf and does not need it yet. She feels a desire to help him.
Lyddie feels sorry for Ezekial because he is one person worse off than she is. Although Lyddie has lost her family farm and is on her own, she still wants to help a runaway slave. Lyddie shows compassion for Ezekial but helping him is a way of feeling better about herself. She feels that she has nothing.
Throughout the book, Lyddie fights an inner war between feelings of selfishness and her desire to help others. Lyddie is basically a good person though. She is tenacious and compassionate. In the end, she always desires to help those in need.